<font color='#000000'>I've noticed the exact same thing with my new 656a. (I have a Philips FR975 receiver.) While I don't own the Tchaikovsky disc you mentioned, I experience the low-volume problem on every one of the dozen DVD-A discs I have played.
(In fact, I nearly blew my speakers out last week when I switched to my CD player without remembering to turn down the volume first.)
While I'm thoroughly impressed with DVD-A fidelity and the surround mixes -- even blown away, in some cases -- the volume issue has been somewhat disheartening. As has been the bass. The 656a claims to handle bass management, but there's still a quite noticeable lack of bottom end. I have to activate the "LOUDNESS" setting on my receiver to get anything half decent.
Would love to get answers on both these issues, but Pioneer tech support is notoriously lame. If you learn anything, please post it here...</font>