Nick, any standmounts are limited in bass response due to cabinet/driver size not just the Castles, these speakers are rear ported to boost their performance (and it certainly works for the Cliftons I use in system 2), depends what you want from your speaker system, I think adding the subwoofer to mine was the best upgrade I ever done did, better than the Primare A30.1 amp upgrade, quite simply you cant appreciate what a sub brings to the party until you try one. For example I was happy with the Rich 2s that had about 55Hz of bass, then adding a sub that went down to 30Hz trasnformed the listening experience, even now I have the sub set at 75% of what it can do and its still stunning. Adding the sub also affected the mids & highs in some way I find hard to explain. Ian J on the sub forum is a good dude to read, hes forgotten more on subs than I can remember. IIRC your A20 mk II has pre-outs so its quite painless to add a sub at some time in the future. I would recommend something like a £275 BKE XLS200, heard nothjing but positive reports. But get some quality standmounts in first, you never know Cliftons with port loading may be bassy enough for you.