In theory, and for all intents and purposes.... digital is digital...whether it's a $20 sound card or a full featured interface costing hundreds of dollars.
In referring to the interface, you said:
But are we " forgetting " how it may sound ? Probably the most important part !Simply put, the interface should NOT impart anything of itself into the "sound" you hear. It should simply be a faithful replicator of the sound you got from your sources coming in. Nothing more..... nothing less.
However......
The differences in sound from one interface to the next would be in the preamps and the output amps circuitry. Obviously, the circuits are similar but not exact, therefore one could expect to "hear" a slight difference from one brand to the next under really good conditions. Going a step further, the studio speakers make a significant difference in the sound heard, but mostly, the person operating the system makes the largest difference in the sound quality with all other things being the same.
If I'm not careful, and paying full attention to details, using the same exact studio system here in my humble studio, I can produce some good sounding stuff or stuff that needs lots of work (after the fact) to get it right.
I use a Focusrite Saffire firewire interface. I've had it for 7+ years now and have run it on 2 different computers. It's solid, and sounds good. To me, "sounds good" means it doesn't seem to color the music I record.... it typically sounds a bit drab or flat with the raw tracks before I add EQ/FX or when I drop all the FX and EQ into bypass mode. Similar to how you described it below with less bass and treble. However, I for one, don't want the interface to give me a biased, colored, fat bass and crisp highs....I just want it to record a flat, unbiased, true representation of the sound coming in from the mic or guitar amp. I will use FX in the tracks and busses to get the EQ right or record it again.
In the OP you said:
First thing i noticed with the 18i8 was...., it is less sensitive on the inputs ( need more gain ) on the input stage. it had less bass and treble, compared to the Ci1 and it sounded more mid ranged and " clean " compared to the Ci1 more " saturated " sound. Were you aware of, did you install, and did you properly adjust the software control panel on the Focusrite? Your input levels are set in it, as are the output levels. With the settings I have, there is more than enough gain in the preamps. I rarely run input levels above 50% or so on the guitar or the mic signal front panel knobs. I don't recall the levels I use inside the software control panel since it's been years since I needed to tweek them (both in and out) but I know it's low (40% to 50% IIRC) because when I was setting it for the first time..... yeah, let's just say, that control panel can really push the volume over the top if you're not careful. I also have the studio monitors set to the 50% detente on the volume control pots. So nothing in my studio is cranked, and there it plenty of upside room should I find it necessary.
As far as the bass, mids and treble..... I would expect the Focusrite to be "clean" sounding as you mentioned. If there is an exaggerated bass or mid or treble response, that would NOT be a good thing in a quality interface used for recording. The last thing you want is a defaulted biased response or perhaps the correct word is coloration of the sound. You want to hear what's actually being recorded to track without coloration. You want to hear the strengths and the deficiencies in the music you have recorded.
With all other things being equal, the biggest cause of bias in a studio (aside from the engineer) would be the speakers. When I went to buy my studio monitors, I did a side by side comparison of the available brands with the same musical reference material....and yes, there is quite a difference in the EQ response across the manufacturers and brands. Some are "flatter" and some do exhibit certain frequency biases. My only advice there is to buy what you like and learn the response curves of your speakers well.
Focusrite makes some really nice quality interfaces. The preamps are pristine.... perhaps transparent would be a better way to say it. I've heard a few guys in the Cake forums over the years complain that the Focusrite pre's were "too clean" and that they wanted some coloration in the signal....especially for vocal mics. The advice they were given by others, was to purchase specific mic preamps, generally tube based, that were known in the industry to give a "warmer" tone. Then, simply run that into the Focusrite's transparent pre's properly adjusted and let it roll.
I don't think you can go wrong with Focusrite. Just my opinion..... However, there are also a number of other fine quality interfaces available that will give you good value and service for your money.
OK that's my 2 cents on this issue.