@Soundblend
In the digital realm, I've had these...
Akai DPS12 - did all the mixing on the Akai and could export data to CD (16 bit)
Tascam 2488 - 24 bit and could export data via USB, but not real time
M-Audio PCI Card - First in the box recording (windows xp) I wasn't very good at it and mainly continued to use the Tascam to record and the PC to mix.
Line6 Guitar Interface - Useful for practicing on the road.
M-Audio Fast Track Ultra - Started to really record in the box. I used the spidf to sync to the tascam so I could record 14 tracks at once.
RME UCX - Now I'm up to 16 tracks at once sync'd to the Tascam.
 
Reason for most recent change:
RME will grow with me over time and works well for live field recordings. Previously, for field recordings, it was the Tascam only. Now I can use the RME with the Tascam and I have done so about 6 times to-date. I've made maybe 20 field recordings with the RME only (that may be an underestimate).
 
Are mixes better?
That's subjective, but I'm able to capture more tracks now in the field.
I think I've learned more about how hardware works and what you might want to do with it by playing with software plugins. 

I use a TC Konnekt 48 and Tascam UH-7000.  They are bang for buck units when they came out.  The 48 I got at 1/2 price because as good as the hardware was 6-7 years ago the drivers sucked.  They are fixed now.   I use the Tascam because it is a very nice converter (and even better preamps).  Yes, I can hear a little difference between the two - esp. the preamps.  Again, the Tascam drivers have a lot of latency, but I can use the digital out to the TCK now if I need lower latency or more in/outs at a time.
 
I had a presonus Firepod before, which was one of the first multi-in/out interfaces that was cheap.  I could hear the difference between it and the TCK 48, although it wasn't as much as I hoped for, and most of it was in the preamp, which was a half step up.  After a few years I upgraded to the TCK since I got such a good deal on it.  And then the Tascam since it is a top shelf converter at a good price.
 
A better interface helps mix, since you can hear what you are doing.  Same w/ the monitors (and room w/o any major flaws).  Plug-ins are good, good plug ins are great, but once you have a tool set you probably aren't going to do much better by switching to X Y or Z.  You are most likely better off learning to use what you have better than switching to something more expensive.  A lot of what plugins try to do is give that analog sound.  It is better to spend money on decent analog once you have some good plug ins.  There is no way to replicate good analog going in (or out) in the software world.  Or I haven't found it.  Good analog going in will be easier to make sound great in a mix that slapping 16 digital effects on a mediocre capture.
 
@

@gswitz, thanks for your input, nice to hear what you have used and learned
yepp can't get enough of inputs always needed, especial for larger projects
in the studio, the band at the pub or on a concert.
btw, thanks for reminding me about RME

to others,i would like to hear some of your experience's

thank's @AT , I'll check a bit, on the Tascam UH-7000 and the other

Jan

RME scales nicely. You can use 3 distinct units together at the same time and each with 18 inputs (just talking UCXs here). So that scales up to 54 tracks at 24/48. Every track has it's own compression and EQ. Each interface shares a verb and delay on a bus.
 
Pretty scalable.
 
And portable!

Soundblend, IMO the monitor speakers and studio acoustics are about 100 times more important for getting good mixes than the audio interface (as long as we're talking about somewhat decent quality, even though not yet "pro").
 
As mentioned above, "different" is not the same as "better", so you simply pick the one you like.
In your comparison there was that one important variable - your stereo amp. Would the result be the same, if you connected directly from interface to active speakers? Could there be differences in the output impedance of the interfaces, one would match you amp better than the other? Just speculating.

I agree that "different" do not mean better ;-)

I am not using any powered Monitors yet, i use :

Sony STR-414L and Dantax Century Main ( 150 watts ) speakers

The system " sounds " better than the active 8" KRK monitors
those monitors are boomy / boxy at ( 180 hz and up to 900 hz, just about ! )
so i will certainly not buy KRK's for sure.
(no offence to KRK owners, but it is my personal ear and taste )

Many of the small monitors have the " in da box " sound, because of the small cabinet
the damping factor, the build material and its own resonance, just to mention some factors.

probably not the Genelec monitors .........  , but i can't tell.

So ( my monitoring ) for now, is better than many of the small " Monitors " out there.

Maybe ill get some active monitors later by Yamaha, maybe the HS8 ?
the ears is the judge, to decide what it will be.

The output impedance on the Audio Interfaces i don't know
but here is the technical data :

Yamaha Ci1 and the Focusrite 18i8 

Anyway there are differences in hardware as it is in software
that apply to Audio interfaces, as it do for EQ, compressors limiters, speakers, amplifiers and so forth
some less, and some more in various ways.

So ill ask this question again :

1. what Audio Interface's do you use , and why ?

should not be to hard to explain, and i think besides monitors and the room itself
Audio interface has to be the interest of discussion, since it is one of the important
element in the audio production, besides preamps, right ?

Jan

I've owned 3 brands now. M Audio Fast track pro, Tascam us1641 and now a Focusrite scarlett 6i6. 
Output sound quality for music playback as far as I can tell has been equal. I've never A/B them but I certainly get used to listening for day and days over the years. 
The M Audio was by far the lowest quality and had a hi pitched scream in the background. The pre amps were also not great but this was an entry level device so what to be expected.
 
The Tascam is a bargain with above average sound quality for what I paid for it. The pre amps are rock solid and I miss using them compared to the Scarlett. But the drivers need improving to get any work done with Sonar. Sound quality wise I really like it and I  am looking at the new Tascam UH 7000 but am still leery of the drivers.
 
The Scarlett works great for me but I'm not using it's pre amps, I use my Yamaha 01v via the SPDIF. I'm picking up a Joe Meek Pre amp next week I'm hoping that will solve the pre amp issue with the Scarlett. My issue by the way is that they are either to quiet or they clip. The Tascam is way more forgiving and seems to handle most mikes in the way I'm used to a pre amp working.
  
But as said, if the end product of my studio is a CD or a MP3 file. My playback equipment will have no bearing on that other than my ability to produce a mix that translates to the the real world. I have 98% success in that regard and that's using all 3 interfaces and even a Sound Blaster back when. 
 
I buy interfaces based on what connectivity and front panel controls they have. I also base my decision on the reviews of the drivers by other Sonar and DAW users. Sound quality is subjective so I do not factor that as part of the decision making process. In other words until I use the device why would I believe someone elses opinion, it's just an opinion. 

Soundblend
So ill ask this question again :

1. what Audio Interface's do you use , and why ?

 
I purchased the Focusrite Saffire PRO 24 DSP, and more details are in this thread, but big picture items (and some not discovered until after purchase) are:

1. FireWire - I specifically chose this as I have an idle FW connection, so knew there would be no usage conflicts.
2. ASIO aggregation - Programs can access the interface at the same time.
3. DSP - Monitoring mixes can have EQ/compression/reverb applied to them without having to route to the DAW.
4. VRM - Not used this much, but allows for room simulation of headphone mixing for various environments. Some purchase VRM boxes separately, and this has that functionality built-in.
5. Loopback recording - Can record "what you hear" from a Windows perspective when routed properly.
6. Construction/Durability - Built like a steel brick, and I very much prefer physical knobs for adjustment.
7. MixControl software - Complex routings can be created in the GUI for the interface.
8. Suited my needs - I record primarily via overdubbing, so this suited my needs.
9. Drivers - Focusrite seems to take extreme pride in their drivers, and I have only seen a handful of threads on issues, and most were not crucial (they were specific to a given scenario).
10. Forum reputation - I did not see issues with Focusrite products prior to purchase, and although not my initial #1 choice, this unit survived my research, and I am thankful for the feedback from the folks here before I bought it.

mettelus

1. ASIO aggregation - Programs can access the interface at the same time.

It is my understanding that aggregation allows you to use two or more ASIO devices at the same time.  I've only seen this for identical interfaces that support it, and a lot don't.
 
A single ASIO device can usually be accessed by multiple programs if these allow sharing.
 
The help for the Focusrite Saffire PRO 24 DSP says that two can be combined without aggregation :
 
The Saffire Line is capable of Dual Unit Mode in which two Saffire are connected via Firewire. In this mode the two Interfaces become one interface. You will not need to create any aggregate devices...
 
 
 

I've used a M-Audio Delta 44 for over 10 years. Rock solid, sounded good. Dug the software mixer.
 
Then for 2 years while on the road, I've used a M-Audio Fast Track. Rock solid, but didn't sound quite as good. That being said, there're other things to factor in which probably account for the biggest part of the difference, such as the fact that it only had RCA outputs, the onboard preamp...
 
I'm now using a Focusrite Saffire. Rock solid, sounds great.
 
In all honesty, I can browse through recordings made on all 3 set ups and the difference that the audio interface made in any case is pretty much insignificant. 
 
I'm not saying that there isn't, but the reason why I dig the Saffire that much compared to my old Delta 44, has a lot more to do with design and access to features than anything else.