As
@hornetstinger just said, and I said at some length earlier in the thread, a BK sub is the one to go for at £500. The KEF looks pretty, but if you want that size and performance you can get it in an 8" woofer BK sub (the Gemini II) for less than half the price, or you can comfortably beat it with the XLS200 for under £300.
You've got a thing for the KEF Q150 & Q650C - that's fine. They're great speakers.
The Audioquest sub cable (Audioquest Black? @ £49 for 5m)... meh. Save yourself £25 and get something that will work better and be miles easier to install. Go on to Amazon and search for 'Super-thin hum killer subwoofer install cable'. It's about the thickness of a USB cable and better shielded than almost any other sub lead you can buy regardless of price.
Cambridge Audio ULTRA MICRO speaker cable - another meh. For a start, you don't need a micro cable if it's going to be buried in-wall. Who the hell is going to visit; Superman, with his X-ray vision?
Honestly, if you're going to buy great speakers such as the KEFs then you want to get as much power to them as you can, and you do that by wasting as little as you can in the cables. What you need is copper, and a minimum of 1.5mm cross sectional area (CSA = the thickness). That's it. Everything else is window dressing. If you want it to be thin when it comes out of the wall, take off the outer jacket; just leave the red and the white sleeved conductors. When you do that, there won't be that much difference in thickness between regular cable and this ultra thin stuff anyway.
Conductor material, conductor thickness, and cable length will determine how much power is lost in cables in the journey from the amp to the speakers. All cables lose some power; that's just physics. An all-copper 1.5mm CSA cable will lose about 5% over a 10m run. That's an acceptable trade-off of power loss against cost.
I'll tell you what really bugs me though about the Cambridge Audio cable; there are no specs for it. Not even on the CA web site as far as I can see. What gives? Hell, they don't even say if it's copper or the much-cheaper aluminium underneath that silver plating.
It's £3.99/m for the thin stuff with pointless 'benefits' and higher power loss. You can buy better for less. QED QX16/2 is an all-copper install cable (pink jacket - LSZH). It's a thicker CSA and way cheaper too. It's under £1 a metre and will do a better job that the over-priced thin stuff.
Denon receiver - okay. It pulls a decent amount of power from the wall (500W) which is on par with the Yam 685. The only major caveat is that the Denons run hot. That's the output transistors working very hard. Onkyos were another hot running amp. They got themselves a rep for poor reliability. It's too early to tell if Denon have a problem in the making.
Re: buying from one place in case stuff goes wrong. Unless you're going to pay them to do a full install on your behalf, I'm not sure that they can be held that accountable for a bunch of boxes that left the shop.
