Because the reviews talk about the speakers needing a lot of room and having a lot of bass, and while you're looking at better amplification you're also looking at speakers that are even more demanding than the Q2050i. The amplification, while rated higher, is still at a pretty basic level so you really need to hear it for yourself. As mentioned, booming is also an issue with the room and the space the speakers have, so going for an even larger speaker probably isn't going to solve your problem. So in that sense, you haven't really understood all of the advice, which wasn't just about better amplification.
The 684s2 (or even Q2050i) is probably a better match for that amplification, but even then I'd be looking for something a bit better. The rated power doesn't tell the whole story because it also needs to be able to handle changes in speaker impedence, and I'm fairly sure the big B&Ws are very demanding in that respect. In that sense, a better amp that can supply more current but has a lower power/wattage rating may actually perform better. I'd be reluctant to pair the 683s2 even with the Arcam A19 and I'd be looking for something ballsier like the Rotel RA-1570.
It's a mistake a lot of people make. They reason that it won an award so it must be good. I'm sure they are great speakers, but they need to have suitable amplification, and when they won the award they would have been tested with a range of quite serious amplification and in a treated listening room. Maybe the testers can chime in about some of the lower powered amps they tried.
As Dave suggested, you can try mixing and matching yourself and go through an (expensive) process of trial and error if you'd rather just go on the basis of something winning an award in a context completely different from your budget and listening room, or you could go and discuss with a dealer and let them show you a few options around your budget. I would never spend that amount of money without an audition. Heck, in the UK it sounds like some dealers let you audition in your own home, which is what is really needed to see how it will perform in your room, because from what I remember from your previous thread it seemed like the room and placement was a major contributor to the loose bass. Throwing more bass at it won't solve the problem.
To be honest, even the 685s2 might have enough bass and scale for your situation.