Do you already have X2700H? If not then i would consider buying X3700H straight away due to better Audussey version as you get the dual sub eq so individual level/delay set for each. The X2700H will see two subs as one and you have same distance=delay for each, may not be issue if placed equidistant from listener, but is that really going to be most optimal in your room... Also there is quite big difference how much and where the correction is done (XT vs. XT32).
If you would buy bundle deal = speakers and receiver together you get best pricing. That would mean you end up paying 650£ for the X3700H as normally it`s 1100£. But i`m afraid you already have gone and bought X2700H.
You can buy these bundle deals without the package woofers which is important!
When you buy the '3700H with the 350 AV speaker pack as a bundle, you'll make significant savings. Not only that but we also offer the option to buy on Finance. Free next working day delivery to most UK regions is also included.
petertyson.co.uk
It`s tough to say which of those speakers you would like best, but looking the center channels i would put Kef Q650c and SVS Prime Centre ahead of B&W/QA and that would drop these two out for me. In reviews Kef has been step up from QA 3000i range (Q150 vs 3020i). B&W has brighter sound which is not for everyone and the new S2 model is expensive, however the earlier gen 606 is available for 450£/pr at PT and HTM6 is on sale too. Kef has the advantage of UniQ coaxial drivers which many loves (single point source, good imaging, wide listening sweetspot etc.). Without listening i would see Kefs (Q350 & Q650c and Q150 surrounds) as safe pick. If demo is possible then i would want to hear the SVS Primes side by side to Kefs.
We have members with dual subwoofers in ~3x2 sized rooms so your room size definitely isn`t issue! You would be suprised and probably little shocked if i would show what type refridgerator sized woofers member has in similar room to yours.
You didn´t mention anything about the total budget so how much you could spend for subwoofers, either one now and then add 2nd later or right away duals. Quality stuff starts around 500£ for the models i would think for your room. So for two you would be spending 1-1,1k£ minium. We have members who have spent 5k£ for woofers in similar size room to yours so the sky is the limit regarding what type of experience you want.
I assume you live in UK, correct that if not.