A21+ is in da house...

jrunr,

I used to have Aerial Acoustic 7Ts with the Parasound A21 several years ago. I submit it's a very hard combination to beat for the money.

jrunr,

I've heard other amps with the 7Ts that were in a different price category than the A21. I can't say if the A21 is right at the "law of diminishing returns" or not, only you can make that determination. I submit that incremental improvements can cost a lot of money, but once you hear them it's very hard not to want the next level. To my ears it's easy to discern the differences between the A21 and Jeff Rowland 625 S2; however it isn't a night and day difference. To my ears, in my system, the A21 was relatively flat sounding and didn't provide the depth and height of soundstage that the 625 S2 does. I'm a firm believer that you must listen with your own ears and that's the only way to decide what works best for you. 

WC is currently comparing $58K and $80 K mono-amps. Both are way out of my price range and unrealistic for most.
The question is what do you prefer in your price range? The only way to find out is through listening. I've listened to components from $2K to  Absolare mono-amps at $70K. Again, there are easily discernible differences,  but at a certain level I just haven't heard night and day differences.