-10dB is still not going to be all the way to 100% on your Adcom. Even with the Rockets' efficiency rating taken into consideration, you'd have to do a lot to truly max out a 125W amp. Going from the Adcom (125W per channel) to a Model 7500 (200W per channel) is probably going to be similar to my upgrade from a Model 750 (165W) to the 7500 (see
my 7500 review for details) - you're likely to hear a difference, and possibly more of a difference than I did, but you'll still run at -10dB on the 950 to get to your preferred movie volume. After all, that volume is based on the reference level at which you calibrated your system.
The 990 doesn't add a lot in the way of specific processing modes. Pro Logic IIx is added, and Cirrus Extra Surround is dropped as a result (PLIIx being better than CES at doing the same thing). Dolby Headphone and a headphone jack are added, as is DTS 96/24 (which I don't think I've ever had the opportunity to use). I'd classify the 990's DAC's as being better than the 950's (my
990 review touches some on the sonic differences that I found in the move from 950 to 990). Remember also that the 990 is
large: it's the same height as a Model 7500 (7.75") and just about as deep as well.
For what it's worth, I wouldn't rule out either upgrade. Not having used that Adcom amp, I don't know how much difference the 7500 might offer, so it could be worth a try. I do know that I definitely got some satisfaction from my 950-to-990 upgrade.