I think the point has been made, but I will use the illustration I commonly use to further the point.
If we consider a £500 stereo amp, the cost is £250 per amp channel.
If we multiply that up to a 7 channel amp, (7 x £250) the equal in an AV amp is £1750. And that doesn't take into account the extra electronics contained in an AV amp. So, more accurately, you would need a £2000 AV amp to equal the sound quality of a £500 Stereo amp.
Or a +£1000 AV amp to equal a more common £250 stereo amp. That puts everything into perspective.
As to Onkyo, I can only repeat the rumors I've heard. Onkyo are stellar for Movie, but just OK for Music. As other have said, there are more musical AV amps out there. But, let's keep some perspective, the Onkyo 818 should do a pretty fair job of it considering it is priced at about £1000.
Also, we assume you have consider all the OTHER standard things. Are you absolutely sure BEYOND ANY SHADOW OF A DOUBT, that the speakers are wired properly. That in every case the Amp(+) goes to the Speaker(+). Nothing will suck the life out of speakers like being wired improperly.
Give some thought to speaker placement and room acoustics. Speaker need a bit of room. There is no need to shove them up against the walls. Most can get by with 10" to 12" behind them. Though I had one pair of floorstanding speaker that really needed a minimum of 18" and more preferably 24" from the wall behind. That is uncommon though.
A simple look around your room will tell you if it is highly reflective. If this is an ultra-modern minimalist room, then it is probably very reflective.
Do you have a specific budget for this upgrade? As has been pointed out Marantz amps are noted for having decent musical capability.
As to adding a stereo amp, this can be accomplished two ways.
One has already been discussed. The Stereo amp can be connected to the Pre-Amp of the AV Receiver, and substituted for the AV amp front channels.
The other method, is to simply use the AV amp for Surround Sound, and when you want music, simply switch the front speakers to the stereo amp.
In this last case, you have two completely independent systems that simply share the front speakers. Beresford makes a quality affordable Speaker Switch that does this very task nicely. It allows a choice of two input sources to feed, I believe, your choice of two speaker systems.
I don't want to discredit the Onkyo to much, there are more musical amps, but at £1000 the 818 should still do a pretty fair job of it.
I have a couple of low cost Onkyo Stereo amps (~50w/ch), and at the price I can't complain about them. I think I like my Pioneer Stereo and my Yamaha stereo better, but the Onkyo still get the job done.
Once more thing, the more AV decoding circuitry you allow in to the system, the more it is going to compromise the stereo music sound quality. If your amp has a Pure Direct or Surround By-Pass mode, try using it to get an idea of how much difference it makes.
Also, for stereo, try using Analog inputs. If you are using a BluRay player as a CD Player, connect the analog out from the BluRay to the amp, and compare the sound of Analog vs HDMI. With HDMI, the DACs in the AV amp are doing the decoding. With analog, the DACs in the BluRay are doing the decoding. One may sound better than the other. Still, we would expect the DACs in the Onkyo 818 to be pretty good.
Just a few thoughts.
Steve/bluewizard