I own and love an Onkyo A-9555 and strongly recommend it for your situation. The price is totally right and you'll probably get your best frequency extension out of your list of candidates. It puts out about 100 wpc into 8 ohms and near 200 into 4 ohms. What seems to translate into a very smooth and dynamic presentation, however, is its instantaneous current delivery when called upon. It can hit instant current peaks up to 80 amps, which is very high. This translates into always delivering the required voltage regardless of the speaker's impedance fluctuations. The A-9555 is also very quiet, so it's particularly good at matching with high resolution speakers, and brings out the dynamics at both the loud and soft ends of the spectrum.
In the Stereophile test, the S-520's tested sensitivity was 84.5 dB. Pretty low, but not so bad in an 11'x12' room. The Onkyo should enable your Ushers to hit clean peaks of 105 dB in an anechoic chamber. This means it would be louder than that in a small room.
Most important, the Onkyo is amazingly transparent and fast, yet smooth and musical. It doesn't sound like a $500 amp at all. I easily prefer it over a Cambridge 640A or Rega Brio 3, both of which I've heard extensively. 3 years ago I installed a Cambridge 640A in my neighbor's house. If I had it to do today, I would get the Onkyo, no question.
In the Stereophile test, the S-520's tested sensitivity was 84.5 dB. Pretty low, but not so bad in an 11'x12' room. The Onkyo should enable your Ushers to hit clean peaks of 105 dB in an anechoic chamber. This means it would be louder than that in a small room.
Most important, the Onkyo is amazingly transparent and fast, yet smooth and musical. It doesn't sound like a $500 amp at all. I easily prefer it over a Cambridge 640A or Rega Brio 3, both of which I've heard extensively. 3 years ago I installed a Cambridge 640A in my neighbor's house. If I had it to do today, I would get the Onkyo, no question.