I absolutely would not keep the BS-22's!
My logic is simple - the Sony Core cs5's are $120/pair and better speakers.
While I have not heard the towers, I expect they are also a step above the Pioneers.
But if it were my money, I'd get 4 ea. Sony cs5's and a good sub. The main thing the sony towers get you is a bottom end to 45Hz instead of 53Hz for the bookshelf. You will cross either to the sub at around 80Hz, so put the savings into a better quality sub!
Of course the Sony center is a good option, but if money is tight, I'd skip the center for now and get the better sub. You can easily add a center later, but upgrading a sub will cost more (because you generally won't get your money back selling a used sub).
I went 6 years using a phantom center (your Denon will automatically configure this when you go through the setup program with no center connected) and when I finally decided to pony up for a center, it was okay, but I was glad I did not make it a priority!
Right now, the Sony's at $120/pair are the hot ticket for a value system. They were competitive speakers for their original price of $220/pair, but nothing can touch them at $120!
I think this saves you enough that you can get a pair of Speedwoofers!