The Clarett series are very nice for recording, but is overly complex for REW.
What you (and I) want for REW is a very simple 2-in/2-out USB sound device with phantom power, flat response, low noise, and up to at least 96KHz sampling frequency (for other things besides REW). No fancy DSP effects, mixing, aux sends/returns, none of it. Just 2 ins and 2 outs (especially for Mac users), front panel gain and phantom power, and USB powering (no external wall blob).
Why this simple? I've tried several complex interfaces, and all they do is get in my way. I work on both Windows and Mac platforms, the multi-channel Mac/Soundflower work-around is a royal pain. I need something I can just plug in a go. In the last week I spent several hours trying to get my Motu 4Pre to work at all, then to work right, after it was working fine the last time I used it with REW. Of course, all of this after discovering it's failure when I really needed to use it with REW. The issues were mostly to do with all of the internal mixing/routing, and some driver issues. So my reaction is, simple is better. Simplest is the best.
So I did up this short list. Tried to stay within the parameters above, there's a couple of exceptions:
The Focusrite Scarlet 2i4 - seems a pretty good match, though I've heard somewhere of driver issues, that may be solved by now. In fact, the only downside to it is the 4 outs, which complicates life for a Mac user.
The Presonus AudioBox iTwo - looks promising, hits the requirements, and is bus-powered for both PC and iOS. Might must be the winner.
The Tascam US-2x2 adds midi, but partially fails by requiring external power when connected to iOS devices.
The Roland Duo-Capture EX is another contender, has bus power, external and internal battery power (nice!) and midi, fails at only 48KHz, but would be fine for REW.
Steinberg UR22 looks interesting, up to 24/192, midi, bus power. No iOS compatibility, though.
IK Multimedia iRig PRO DUO - very compact! Includes midi, bus power and battery, PC and iOS compatibility, limited to 48KHz.
IK Multimedia iRig PRO - same as above, but 1 channel an up to 24/96. Tiny! I'd like two inputs, though.
I'm sure there are others, but so far, the Presonus AudioBox iTwo seems to be the closest to ideal. I don't know if it's been tested with REW, but I don't see a reason it wouldn't work.