What Power Amplifier Should I Buy?

Once the A21 warms up for a couple hours it just sings. I prefer it to the $6K Coda No.8.

I never heard the A21, but I was very impressed with the A21+. For the cost it was an excellent amp. I sold that to get the CODA #8 and thought it was more detailed and as powerful as the A21+. I now have the CODA #16 which is better than the #8. The Benchmark LA4 with the Parsound is a great combo. The Holo Serene preamp is another similar option.

Using a stereo amp as monos is not a good idea. Better to get dedicated monos. I would get the stereo 4B3 over 2 2.5’s. A lot of technical explanations on Audiogon about why this is the case.

I am selling my Sanders Magtech amp (500 W) which I think is as good and maybe preferred over the 4B3. Definitely great on Maggies but I am not sure about the Revel and Sanders synergy. My Sanders was not as good as I expected on my Yamaha NS5000 but on my Magnepan LRS+ it was amazing.

The 4B3 should be good on Revel. I used to have the 4B-SST and then the 7B-SST on Revel Salon 1. It was not perfect but not too bad. The extra power of the 7B-SST was noticeable on the Salon 1. Such killer bass on that speaker.

Orchard Audio Starkrimson Ultra DMC 2.5 is what you want. 

Agree with YYZ I would not use the 2.5s in mono, but as I recommended in my post if you use them to vertically biamp your speakers (since they are biwirable) there are many advantages to that over a larger stereo amp.  

Get a second HSU sub no matter what. Get one before making an amp change because two-four subs is where you want to be.

I`ve heard the Revel 208`s many times used with 3 HSU subs.

Running the subs up to 120 Hz is only going to muffle/muddy the bass up.

Use 60-70 Hz and play with Q settings and volume level.

i have just the amp for you in your price range, recently retooled by jeff at conrad johnson, i dont play it anymore because i sold the preamp.  look under audiomart under conrad johnson, but its a mccormack dna 500...... 500w the 2nd best amp ever made.