Oh, just re-reading and saw your alternatives. My comments:
Cambridge Azure 851 - Cambridge is known for being lightning fast and extremely transparent/resolution. I would not pair this directly with the Oppo, but it may be a good amp if I had a tube preamp or a multiple-circuit Class A preamp.
Audio Research 150.2 - This is a Class T design from 2003 (which is a variation of Class D). It might be nice, but the older Class D type designs were not as good and the resultant frequency response was very dependent on the impedance curve of the speaker. This translates into un-even frequency response, possibly weak midbass/bass, rolled-off highs, etc. Also, it is not a Class A bias (obviously), so it is not going to be as sweet as Parasound.
Parasound A23 - Like I said, if you have to get this because of your budget, then get it. The larger A21 does have twice as much power output, but you’re not buying it for the watts. You a buying a significantly larger power supply and more output transistors. This will mean more brute force to control the speaker -- even at low to mid volume levels. Also, the larger A21 will just be a more refined and smoother sound than the A23.
