given your budget range, and if you really do like your 555ii, one option is buy another one as you're contemplating, but rather than run it in bridged, run each of them in "single stereo". This is where you only use one channel of each for each speaker. I'm not familiar with the Adcom, but believe it's using only one PSU for both channels. Running only one channel buys you more current supply, and effectively more power. Parasound claims their HCA1000a drives 40% more power in single stereo vs rated stereo output. Of course, if you bought the second, you could try the bridged mode, but I would be really careful with impedance dips.
A second option is to sell the 555ii and find a pair of 656 mono blocks.
In something not Adcom, a Parasound HCA2200 (mk I or mk II) can be found used a little higher than your budget, but selling the Adcom will cover this. Both are true dual mono (dual transformers), ultra high current, rated at 240 watts, and drive super low impedance. A HCA3500 may be found for $1100-1200 putting it out of your range probably
Finally - - these B&K M200's may be worth a try. Rated at 240, with tons of current and low impedance stability. I'll be surprised if the auction tops $500 by much is at all.
A second option is to sell the 555ii and find a pair of 656 mono blocks.
In something not Adcom, a Parasound HCA2200 (mk I or mk II) can be found used a little higher than your budget, but selling the Adcom will cover this. Both are true dual mono (dual transformers), ultra high current, rated at 240 watts, and drive super low impedance. A HCA3500 may be found for $1100-1200 putting it out of your range probably
Finally - - these B&K M200's may be worth a try. Rated at 240, with tons of current and low impedance stability. I'll be surprised if the auction tops $500 by much is at all.