ProAc Response 1SC vs. Revel M20

These are two very different speakers. There are other options in this price range. If you consider that the 1SC need very good stands, your budget moves to about $26-2700. But, the real issue is what are you seeking to accomplish? What is in the rest of your system? And, what is your room like?

I can speak for the M20's as I have owned them twice. They sounded their best when I heard them with a Levinson integrated. I never owned it, just heard it demoed. They can be a little tough to drive, but are very worth it. The bass is very good for the size and the imaging in incredible when they are set up right. I owned the M20's with a few different amplifiers and the match I prefered the most of what I owned had to be the audio refinement complete integrated. Warrenh used to use the M20's with a YBA. Which is in the same family as the audio refinement.

The Revels are more linear in spectral tilt and thus require more attention to sidewalls, room dampening, etc. They're VERY carefully cloned, so imaging can be spectacular. The ProAcs are fun, and a bit more forgiving.
I like the M20s better in a high res system. Yeah, there are many other choices. I use Spendors for HT because they're even MORE forgiving of unruly top octave broadcasts and soundtrack mixes. But for 2ch high res, especially in the nearfield in a good room I'd go with the M20s.

I had the 1SC's for a short while. I also heard the M20's on an occasion or two in other systems. My impression was that the M20's were a more complete accurate sounding speaker. My impressions of the 1sc's was one of warmth, with nice midrange, but not so gutsy in the dynamics department overall, more so down in the bass, which was more limited. The M20's struck me as more of a high end effort through and through.
A lot I suppose would have to do with what gear you were using with them, I suppose. I think the 20's would be harder to drive than the 1sc's, which might do well with even lower powered tube gear in the right system...but that's an assumption. (I used SS with em at 100w/ch)
If I had to, I'd lean towards picking up M20's to play with in my system personally. I wouldn't bother again with the 1sc's really. But that's me.
Of all the Proac's I heard, I liked the 1.5's the best(or was it 2.5?...I forget. Whatever, it's the smaller floor standers with one mid/bass driver).
As for room placment/setup needs, ANY SPEAKER NEEDS THIS!...it's critical for best sound.