Scoly, I have to disagree with a previous post that recommended you upgrade your speakers first. You've already tried that and got your answer, I think. If you have anything to spend, spend it on a better amp and preamp. Buy used if you have to, to get better sound. I have Acurus 125 x 5 and Act 3 for home theater (upgraded from a Pioneer Elite receiver) and am dazzled by the improvement on CDs just playing in two-channel. The differences you are looking for--liveliness, detail, air--are there on music even though my speakers remain small Boston Acoustic monitors (VRM-60's). The Acurus is cheaper than a lot of stuff, but not dirt cheap, even used. Sell your Denon for as much as you can, and use the proceeds, plus anything else you can scrape together, for a good amp and preamp combo. You can change your speakers later, they really should be the LAST to go, in my experience and opinion.
Sarah
Sarah