Amp recommendation and setup advice please

There are other subwoofers you can get.  Look at these on the ebay UK site:

https://www.ebay.co.uk/b/Speakers-Subwoofers/14990?Configuration=Single%2520Subwoofer&rt=nc&_sop=3&_udhi=500

There is a Revel B120.  Also, several BK subs so that you could get two if you wanted.  Or just get one to start and put it as close to the corner as you can get.  If you get two subs, use Left RCA for one sub and Right RCA for the second sub.  If you only get one sub, use both left/right RCA into the same sub.

You can play with the Yamaha's position.  I think generally, you need a little bit of space away from the wall (at least 1-2 feet) because the bass ports are on the back side of the speakers.  But I think the best bet would be to put a subwoofer in each corner. Then set them at about 80hz (the Yamaha's may not play that low when in the middle of the wall).

I can understand your concern on the "split" of the Audioquest cable.  It is definitely not that wide, and the left/right RCA outputs on the Audio-GD are spaced far apart.  I would look at the Belden 1695A Coax cable for subwoofer.  These come in single cables, so you can get two of these (one for each corner sub).

https://www.bluejeanscable.com/store/subwoofer/index.htm

The 1695A uses a solid-core 18awg copper conductor.  In my opinion, this would be an excellent cable for transferring bass frequencies.  Smaller awg conductors do not transfer bass as well.  In my testing, 20awg seems to be the best overall compromise - small enough for high frequency detail, but still large enough for good bass.  When going to smaller 22awg conductors, I get much leaner bass and too much bright highs.  The Audioquest interconnects typically use 21awg conductors.

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There are definitely counterfeit cables on ebay and stuff.  The Kimber Cable speaker wire is very easy to counterfeit because you cannot tell the difference - it's just braided wire.  There are other Furutech connector counterfeits.  Lots of counterfeit junk on ebay from China - stuff that is "too good to be true".  However, buying something like used Audioquest on ebay or elsewhere is usually pretty good.  I have not seen counterfeit "equipment".

There is one area where silver definitely makes a positive impact and that is with digital cables (including COAX and HDMI). Silver is a much better conductor than copper and will charge/discharge much faster. This is critical when dealing with digital square waveform signals that are extremely high frequency (like megahertz). For normal audio waveforms, I prefer pure copper.

The other aspect of digital cables is length. It is said that you should really use 1.5 meter or 4.5 feet as minimum length to reduce the amount of signal reflection back to the originating transceiver. Signal reflections can cause distortions in the waveform and cause inaccuracies. I have used short cables and this can be heard as a slight smearing of the audio. I always use 2 meter / 6 foot digital cables just to be sure (toslink doesn’t count here because it’s light).

This is the cable I use:

http://audiosensibility.com/blog/products-2/digital-cables-occ-copper-and-occ-silver/#!/Neotech-NEHH-4200-HDMI-1-4-Cable/p/17895256/category=4059160

I like it because it’s not extremely expensive and he cryogenically treats the cable (which solidifies the conductors just a little bit more and enables more high frequency transmission). The Audioquest Chocolate is also a good choice as it uses silver-plated conductors as well.  I might think the Neotech has a slight advantage because Audio Sensibility does the cryo treatment.

I have wondered about upgrading to something better like Wire World Starlight 7 or Silver Starlight 7, but other things have higher priority on my list and I don’t feel that my Neotech HDMI cable is a weak link yet.

You may or may not notice any difference in your system with upgraded HDMI cable. However, you might notice a slight improvement in video clarity.