What Revel rear speakers to use?

I would be less concerned about the rear channels than the mismatch between your fronts and center channels. The "logical" change would be to replace the C-30 with either another Gem or a Voice from the same top tier Ultima line. The C-30 while nice, is from the entry level Concerta line. As the center channel is arguably the most important-and should tonally blend seamlessly with the fronts, this is the place to start IMHO.

I agree with Spinaker in that the seamless intregration across the front 3 speakers is a priority in HT setup. However the C30 is from the predessor to the Performa F32/C32/M22 line up. The C12 is from the Concerta line up.
To find the rears that match with the C30 and F30 I believe that those were the S30 which were a di-pole design.

I bought a used pair of the original Revel Studios last year and pretty much went through the same search as the OP. After researching it to death and given my budget, I ended up agreeing with Spinaker that the surrounds were not as critical. I got a pair of S30s and am very happy with the results. They can be switched between dipole and monopole to adjust for music vs movies. The only caveat is that they are on-wall speaks and that they do stick out a bit so you should make sure you're ok with that.

Dear Simone,

I can answer your question with quite a bit of experience on this front:

1) I have an all Revel setup that has evolved over time.
2) When I was initially setting up my system, I had contemplated building around Focals, so I had gotten a great deal on Focal dipole surrounds.
3) I then purchased Revel M22 fronts with an C32 center and mated those with the Focals.
4) On their own the speakers were amazing, but there was a difference in timbre with any true surround effects.
5) I then sold the Focals and got the S30s. Perfection. Absolute perfection. I preferred them in monopole mode vs dipole mode.
6) When I moved, I was unable to use free-standing surrounds and had to go all architectural. I now use IC15s for my side surrounds and IC6T2s for my rears.

In your setup, the best choices would be as follows:

1) If you are looking at monitor/direct radiating speakers: Depending on your budget and goals, you can opt to get Ultima Gems for the side surrounds. Those are obviously the perfect match. Optionally, get either M20s or M22s for the side surrounds and put those monitors on stands. You can also look to get any of the new Performa3 models.

2) If you are looking for dipoles, then get the S30s or the matching Ultima Embrace. I would then look at the Concertas for the rears in a 7.1 setup just based on cost or go with S30s for the rears as well. I wouldn't suggest going with the Ultima embrace for the rears unless you had the liquid cash freely available. The S30s are being cleaned out of final inventory and you can get them brand new with warranty as low as $499 each in white only. That's a steal for $2,000/pair speakers that will mate perfectly with your setup.

Of all the options, I personally prefer the S30s. They have dipole/monopole switching and you can even trigger that switching based on source. They can be either wall or ceiling mounted. If you cannot wall or ceiling mount, then go with the monitors.

Internetmin,
I am in dilemma regarding my surround and can surely use some help from an experienced user of similar products like you.
I have F52s, Voice 2 and S30s currently. Planning to move to Studio 2 in near future, within next 6 months or so.
I run these using Integra 80.2 and Sunfire TGA-5200.
What, if any, noticeable differential gain I might have if I replace the S30s with Embrace? On paper, Embrace looks quite a different beast, with two matching tweeters instead of one in S30 and the Woofer is bigger. I think one can run these setting up as "Large" speaker while configuring the crossover. Will that or will that make a worthy improvement for Sci-Fi movie watching? I do Matrix, Lord of the Ring, and Casino Royal kind of movie quite often... Will appreciate your feedback!

I personally don't think your aim is for speakers to go for "large". You would be better off adding subs. Floyd Tool's research shows that the ideal is 4 subs--each out in the middle of the walls for the smoothest bass response. If you want more "oomph" then I think you want to look there. I think the embrace will give you more clarity and transparency and they can also move more air, but not necessarily bass.