What amp do you have now? Based on my recent experience with the ARC SP16, i would start the upgrade path by replacing it. I wanted a tube preamp to use while my VAC Renaissance is in for repair and updates. I brought home an SP16 and it took all of five minutes to decide this was not a good choice, very limited bass, flat two dimensional soundstage, plus the awful stepped volume control. I then took home a Manley Shrimp which comes much closer to the sound of the four times more expensive VAC. It does not match the VAC in bass weight, detail or separation but the overall presentation is one I can live with for awhile. The biggest downside is no remote control but it does have a nice feeling volume control that is not stepped. IMHO, no matter what amp you purchase, your speakers will sound thin and weak in the bass with the SP16 preamp. B&W's need a bit more meat on the bone and just changing the amp won't do it.
Amp upgrade ?
I have a handful of amps in mind that I would like to upgrade to but I can't decide, #1,pass aleph 1s #2,Krell KSA 250 #3,Bat vk-500 #4,Sim w-5 #5,Mark lev 23.5 my current system is B&W 804 pre is audio research sp 16 cd player sony scd-1 I have heard all of these amps but in different system configurations even though some are much older than the others I think the krell and mark are still good units.