I have the Denon 1906 and was originally using them with Kef 105's. I can honestly say that changing the 1005's to the 2005's gave a HUGE improvement. The 2005.2's can be had for just £399 at various places right now. Probably because 2005.3's are supposed to be due out soon.
Haven't heard the 606 myself but from what I've read, I'd be surprised f it was that much better then the Denon for music or even for ordinary DD or DTS. Obviously it has the latest HD audio codecs so it does win hands down for audio quality from blu ray discs.
So in a nutshell - IMO, if you after HD audio then it has to be the Onk, otherwise changing the speakers will give you a much bigger overall improvement to the sound quality.
Heard the Onkyo 606 demoed on some really low quality speakers, and it sounded amazing, partnered with better speakers it will sound better still. I would get the Onk first, speakers will come down in price quicker as new ones come out quite often.