I am not familiar with the tweeters chosen for the B&W 600 series speakers, but the 800 series had metal dome tweeters. These are more sensitive in revealing issues in the front end of your system. In general, the Denon electronics are a tad uptilted to begin with, and if you are serious about Audio, the SONY multi disc player should be the first piece to go. Consider purchasing a single disc, dedicated CD player...with a solid drive and isolated chassis. Sim Audio makes a very nice, musical unit, the Equinox, for about $1200 on the Gon. I would make a move on the front end because you can only get quality out if you put quality into the music stream.
If you opt to upgrade in the future, purchase a speaker with a soft dome tweeter rather than a metal tweeter. Dynaudio 1.8 is an excellent floorstanding speaker and very musical...one was recently for sale. The Dynaudio 1.3 is also nice for a bookshelf, and the Reynaud Twin is a steal, going for $600-$700 on the Gon. Another great line of speakers are the Proacs...the SC-1 or Proac 2s are super musical...and you can probably find the 125S or 150S available for about $900-$1200. These are exceptional speakers for any amount of money.