Buying an additional amp is a good way to upgrade an existing system, but I don't recommended that people buy a new AV amp and add a Stereo Amp to it. Better to simply buy a better AV Amp. Not suggesting what you are doing, simply commenting on the general economics.
The
Marantz SR7008, as you can see from the review I posted, is a very highly regarded AV Receiver with very good Music capability. But, there are limits to everything.
In a really first rate Stereo Power amp, I would suggest the
Parasound Halo A23 with 125w/ch, but at the considerable price of
£1000.
Product Lines > Halo > A 23 Two Channel Power Amplifier
http://www.stoneaudio.co.uk/?product=parasound+halo+a23
You have to ask yourself if you want to spend £1000 more, or if you would rather dump your existing amp, which should gain you some money, and then spend a considerable £2200?
Or, the least costly option, just learn to live with what you have?
If you choose to off-load your existing amp, consider how much you will get for it.
The current Marantz SR7008 are running a bit under £1000, which means you can probably get in the range of £500 to £700 for your existing amp. Is that going to be enough return to make it worth investing in a better AV Receiver?
Marantz SR7009 9.2ch Network AV Receiver
So, the upgrade possibilities are -
1.) Sell Marantz for ~£600, then spend £1600 on the Anthem.
2.) Keep the Marantz and spend £1000 on a Power Amp.
3.) Consider keeping the Marantz and adding one of the AudioLab amps. The current new AudioLab 8200P is about £650 from several sources. The Power is 100w/ch to 8 ohms RMS.
One advantage of a separate amp on the Front Channels is that it draws on its own Power Supply. Most Stereo amps have overrated power supplies - meaning 100w/ch will have a 300w Power Supply. Most AV Receivers have underrated power supplies - meaning if you have 7x100w channels, the Power Supply might be 500w. Though amps like the Anthem have pretty strong power supplies.
The last consideration is the size of your room and how far away you sit. The greater the distances, the more power you need.
In a Pre/Processor + Power Amp system, there are two companies that I know of that make a 3-channel amp
(Parasound, Emotiva). If you are building a 5.1 with discrete amplifiers, a 2-channel in front, and a 3-channel for the Center and Rear would be my personal preference ...you know... should I ever win the Lottery. My preference would be to build a AV Surround system with discrete amps, though that gets quite expensive. But that is a completely different matter.
From the options I listed, you have to weigh which is the financially best path for you.
If you are sure it provides the quality you need, then perhaps the AudioLab 8200P is the best choice in terms of lowest budget. But make sure the amp lives up to your quality requirements.
Steve/bluewizard