Thanks - what would you recommend?
There are two things to look at. The first, ideally you want a stereo amp that can behave live a power amp when used in surround mode in concert with the surround amp. To flesh that out a bit, one upgrade for home cinema amp owners is to add external power amps for the key channels. The centre and the stereo front channels are the main beneficiaries. However, although that shifts some of the heavy lifting duties away from the surround amp/receiver, it still leaves all the audio sources going through the AV amp's pre-amp section. That might not be the best when it comes to analogue sources and some digital sources too.
The solution is to add an integrated stereo amp which does two roles. On its own, it keeps stereo sources clear of the AV receiver's pre-amp section, so when listening to pure 2-channel music there's no AV gear in the signal path. In AV use, the amp provides power for the front channel stereo speakers. However, unlike a power amp, a normal stereo amp has all the source signals going via its own volume control. This means to maintain a proper channel balance in AV use, the stereo amp needs to be set back to XdB volume level, whatever 'X' happened to be when the AV system was set up. That might simply be a certain point on the stereo amp's volume dial. The alternative is getting a stereo amp with bypass mode. This means that there's a certain input or mode that sends the AV receiver front channel signal directly to the stereo amp's power amp section at a fixed level. In bypass mode then the stereo amp acts just like a regular power amp. All the volume control is done via the AV receiver.
Someone put together a list of older stereo amps with bypass mode. I can't vouch for the accuracy, but at least it gives you a starting point for new or used amps. If you were seriously considering the UB9000 at a grand less what you'd claw back on the 820 then we could estimate a budget of £750. That would get you some really nice amps if you were looking at secondhand. Check out the list
here.
The other thing to consider is the power of any stereo amp. Denon AV receivers are pretty potent, but a stereo amp with a toroidal transformer and hefty capacitors doesn't need to match the Denon watt for watt to keep up. Good stereo amps do tend to be strong performers when it comes to sustained power output. I'd stay north of 60W on any premium stereo amps. For mainstream Japanese brands such as Denon, Yamaha, and Pioneer, you might want to hedge your bets with 80+ Watts.
