My Audio Interface IS a mixer... A digital one...Uses an app like bitflipper describes, no hardware faders at all.  In fact the only hardware volume control is on the headphone jack...
 
 

I have 2 mixers, a 24 and a 32 track, sitting in storage wasting away. Once I got to affording powerful computers, the mixers just got in the way of the workflow. I do agree that a great analog mixer is a joy to listen to but the absence of them has hardly been noticable. 

Hi Space, I'm one of the guys that use the 500 series and summing mixer setups you mentioned. My pre's are mostly 500 series and then I go out of the computer into 16 channels of summing along with a stereo eq and compressor in an SSL X-rack. I really like the X-Rack and what it does to the sound, but truth be told it's kind of a hassle. I still have to use session sheets for clients and project recall is sometimes problematic, particularly with all my other outboard stuff.
 
Do I think it all makes a difference? Yep, and I've done the null testing to prove it . Does it make things more difficult sometimes? Yep, and I've gone through the heartburn to prove it. I like it and I'm not leaving it anytime soon, but project recall, session sheets, and repair bills are something to keep in mind.
 
Dean

I use a digital mixer. But I don't mix with it. 
It's sort of a central station everything runs thru.
I think I would miss that convenience but I've looking for smaller footprint alternatives.

My friend uses a really nice old Studer board basically as a summing mixer, or for drum coloration during recording. I think the benefit of a summing mixer and good analog preamps is that you don't have to make/keep session sheets. You can leave everything plugged in, get some nice analog color on the way in and analog summing in any configuration by routing from Sonar (or TotalMix). I've also worked in a big ProTools studio with the latest SSL 4000 series, honestly it sounded amazing but even with its own memory for automation and mix it was a giant hassle added to the workflow. I wish I owned even a quarter of the premium outboard they had but at the end of the day I'm more of a producer than a mixing engineer and I would go nuts having to repatch all the time (and remembering/recording how) while switching between projects.

So 500 series pres and a summing mixer sound great to me, especially since you already have an X-Touch and Console 1 for the feels!

timidi
I use a digital mixer. But I don't mix with it. 
It's sort of a central station everything runs thru.
I think I would miss that convenience but I've looking for smaller footprint alternatives.

I used to do the same thing until I bought a Mackie big knob.
I cant tell you how much easier it has made things. 2 presses of a button and I can go from monitoring on my KRK's to my JBL's. I also send a mono mix in to my drum room through pa speakers and use talk back through that.
At first I had heard the BK colored the sound so was a little hesitant, but I found only if you crank up the pre amps on the BK you get some color. I also have a TC leveling pilot which is totally passive and to be honest I cant tell the difference. I ditched the TC leveling pilot because it only had room for 1 set of monitors. I have 3 (2 for near field / 1 for drum room, recording)