use Dennon 2802 as pre-amp and add tube amp?

Yes, IMO, it is worth a good amp to get cleaner sound. My
experience w/B&W spks(DM 600/640, CDM 2) is that they are
forward and somewhat bright. A warm SS or tube amp might
off-set this and make them more enjoyable to you. I've been
lead to believe that the 300's w/the soft dome tweeter sound
warmer? Try to mellow out your system with warm interconnects and spk. cables. Perhaps consider a change of
speakers. Spendor S 3/5's sound wonderful (I believe video
sheilded also.) Great midrange, would need sub w/HT. Three
S 3/5's as LCR would be glorious HT set-up w/sub if needed!Keep the B&W's 300's as rear spks. Just a thought! Bill

I have the Denon 3300 HT amp and had the same problem.
I tried to use it as a preamp with Mcintosh SS amps and did not like that ether. I went to a tube preamp with two tube
amps and that did the trick. I run a 40 WPC push pull on
Maggie SMG's and a 5 WPC SET on JM Labs opal's. I have not
turned on the Denon in six mounths. RG Knaak

You have to ask the question whether you really want to mess with tubes in what seems like a combo HT/stereo setup. Also, B&Ws are not the best speakers to use with tube amps in the first place as they require more power to really open up than tubes can usually provide(unless you're ready to plunk down big $$$). Personally I think you'd be better served by trying a warm sounding solid state amp, like Classe or maybe even McCormack, or, better yet, consider replacing the speakers. I'm very familiar with the 602s and although they're an outstanding bargain the tweeters are a little on the hot side, which sounds like what you're hearing. I think you're starting to go down a VERY slippery and expensive slope trying to tame your speakers with electronics--try new speakers first. Given what I said above, I actually question whether this dealer really has your best interest in mind here at all. Best of luck.

Tim