Doing a quick search I see there was a pair of Belles MB550 monoblocks listed on Audiogon for $5900. I have the Synergy 450 and love it but a pair of monoblocks would probably give more of everything, that said the Synergy already has more reserve power than I ever use so the monoblocks would be overkill. Now if your speakers are inefficient and in a very large room then more power is always a good thing.
If you are going new then get them both, AVA has a 30 day return policy. The only way to know for sure is to try both amps. I don't know what new Belles 550 amps cost but the Synergy 450 is only $1999 so it's sort of comparing apples to oranges.
