My Scarlett 6i6 has 3 sets of stereo output to do what you want. 2 pair are analog and the 3rd is SPDIF so you can use that for loop back. This is better as there is no D/A- A/D involved as well. 
Focusrite is popular because for the money they have solid drivers and excellent customer support. Your hard pressed to find an negative review. The new Tascams are also worth checking out now they've finally started writing better drivers. 
Mont multi channel interfaces have the same 1/2 -3/4 SPDIF set up.  
Then it depends on your input needs and they come in all configurations. 
Here is my shopping list of features to be aware of when looking at an audio interface. 
 
http://www.cactusmusic.ca...0Interface%20blurb.rtf
 
What kind of connectivity do you require? and how many of each.
These are some of the options:
XLR, 1/4",  Combi jacks,  RCA, ¼” line level, ¼” Instrument level ,MIDI, SPDIF, ADAT,MADI
How many ins and outs do you think you'll need now and in the future?
Are they accessible?  Front or rear panel?
Are all ¼” jacks Balanced?  
Is there a true stereo pair? Some don’t have a matched set of inputs.
Pre Amps will fall under the “you get what you pay for”  rule, but check out the reviews anyhow.
A/D convertors will also fall under the same rule and generally all are good these days.
Are there peak level meters or just a little LED for each input?
Are there channel Insert jacks?
Are there separate controls for Monitor level and headphone level?
Is there a blend control for mixing Source with Computer ( DAW) ?
How many Headphone jacks? A level for each?
Are the input pads or line / Instrument toggle switches on the front, back or software controlled?
Is it a metal box or cheap plastic? Is it light and portable or large and bulky, Rack mountable?
Does it have an on / off switch?
Does it use Buss power or a power supply? Buss power can have issues with noise and Phantom power. Look for at least an optional power supply.  
Does it have DSP effects built in?
Does it use a GUI mixer? Having a software (GUI) mixer adds more options.
Can it be used as stand alone? Some interfaces are also handy as a small mixer.
What are the Round Trip Latency (RTL) specs? Do you need low RTL for real time processing?
Low RTL is going to be at a higher price point. A $200 interface will have hidden buffers etc.
Zero Latency monitoring is not the same as RTL. All interfaces have some latency.
Zero latency is just marketing hype for monitoring directly from the interface.
And most important of all, Does it have top notch drivers for your OS. 
What is the word on support from the company?
Does it come with free software, An LE version of a  DAW you would like to try?
 
Everyone will recommend the interface they have chosen, that doesn't mean it is the right one for you. 
 

I didn't think about the loop back using SPDIF. That is great.
 
If I have to upgrade, I just don't want to go backwards.
 
The Profire 610 can sample up to 192KHz but many of the interfaces in my price range only go to 96KHz. Having said that, I have never tried it above 48K.
 
The Profire does have a built in DSP.
 
BTW I contacted Focusrite and asked specifically if their FW audio interfaces would be supported on Windows 10 and they said yes and the drivers will be be ready by the time Windows 10 is out. So that is good news!
 
The only other thing I think I need to figure out is the sound quality of other devices compared to the Profire 610. It sounds like for lower end devices (like the ones in my price range) are not the greatest. I wonder where the 610 fits. I do not want anything worse than what I have.
 
Oh yeah, I heard that some interfaces are noticeably noisy with dynamic mics due to their lower output. I have seen the SM7B mentioned a few times. I don't have one. I do have SM57 but they get driven pretty hard in front of an amp. Either way, I plan on setting up the Profire to send the mic inputs out the back so that if the driver no longer works in Windows 10, I can at least use it as a mic preamp.

I did a lot of research before I got mine, and got a lot of bang for the buck for my needs. I chose FW specifically to avoid any potential USB conflicts, but the Saffire Pro 24 DSP also includes their "VRM box" internally as well as audio loopback path (also internally wired). These features do not exist on all models (I *believe* the audio loopback is only on the FW line, not USB). Another thing to adjust is the sample offset to any AI (this one is a manual 76 samples for 44.1/24). Downside is I am limited in I/O (I overdub solo mostly), but appreciate the connection setups/knobs on this unit a lot. Focusrite seems to take their drivers very seriously (I do not have experience with other AIs, but am keen on watching thread titles with problems), so I do not expect legacy support to cave over time with units still in the field. I *assume* that their MixControl software is updated to include all models it was originally written for anyway.

Focusrite Saffire Pro 24 DSP no longer exists. They don't have VRM anymore according to their website. That is, no products come up when you choose that option. I couldn't find out why.

Had to rewrite this completely - Okay, that comment made me research. It looks like the 24 still includes the DSP (just not printed in the title anymore), and the VRM was removed from the unit (which I have not used anyway, TBH). Thunderbolt support is new (I think).
 
It might be worth asking Focusrite directly what the difference is between the new and old one. The Red 2/3 plugins came out after I bought mine and were given to owners (and included now).
 
If the only difference between new and old is the "VRM" part, then it wouldn't be as earth-shattering as I initially thought here (I believe the 24 was the only model to have VRM built in, but not sure).
 
Edit: I just submitted the question to Focusrite and linked this post.

Thank you!
 
BTW I was researching latency issues and found a site that did benchmarking of interfaces and it looks like the Focusrite units tended to have the worse latency. My Profire 610 came out pretty good. I am not sure when these tests were run but I read various forums posts around the web that Focusrite has improved the latency through driver changes. 
 
One youtube video indicated there was a setting in his driver where he could adjust an internal buffer and he changed it from the default to something lower and was happy with the results.
 
I am a guitar player and while I don't love sims, I do use them from time to time and too much latency is hard to use with an amp sim (at least for me).
 

I went from a Firewire unit, a Focusrite Saffire Pro 40 (after it died out of warranty), to a USB unit, a Roland Octa-Capture and my latency dropped in half on the same PC.  The best I could get (for reliable performance) was 11ms with the Saffire Pro 40.  With the Octa-Capture 4 to 6 ms.  I now have a Roland Studio-Capture and it too is the same latency as the Octa-Capture.
 
My Rolands are Win 7/8/8.1/10 compatible.  I have Win 10 Preview (pre-release) on my backup system, on a separate hard drive, and have been testing my recording Software (Sonar Platinum) and hardware.  So far, everything is working properly.  Even the old Frontier Tranzport works in Win 10 (it has to be installed in a Win 7 compatibility mode, but it had to be installed that way in Win 8/8.1 too).
 
 
 

Yes, Focusrite interfaces do have worse latency than most (as do Presonus products, for the same reason). But it's due to the chipset they use to implement I/O, and has nothing to do with Firewire versus USB. 

Many Firewire devices in audio equipment use the DICE or DICE II chipsets, including Focusrite.
 
This is one reason to use a T.I. firewire chipset in a PC; the DICE chipsets want to see a T.I. and usually only work or work properly with a T.I.  I haven't got into the electronics/logic to see what specifically why (I'm an electronics tech but not an Engineer so a lot of what is available is useless to me).

My TC Konnekt FW unit dropped the latency in half (from 12 ms to about 5) w/ the latest drivers.  They say they are still working on drivers for Win10.  They use DICE (I think they developed it IIRC) and it works fine on my old VIA chipset FW card.
 
As far as sound, almost all similarly-priced units will sound pretty much the same.  They basically use the same components, same IC preamps.  The good news is they all sound very good.  Worry about the software, not about a $5 preamp sounding better than another one.
 
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