Considering new amp - Mccormack

DNA-500 would be my choice. I have 1 that has been replaced by tubes, but I'm keeping it. Best bang for the buck at around your price range, used, that I've found. Steve McC would be the 1 to talk to and I would also check out the thread referred to above for more info. Good luck

I respectfully disagree with Tvad that Radiohead is recorded "hot." I think their recordings are amazingly well conceived and engineered, in the vein of all great Brit-pop. Especially "In Rainbows." I think you need a very musical system that can reveal the intricacies found in their recordings.

System synergy is a big part of all this. I'd venture that the Adcom amps coupled with the BWs are a lot of the problem--the grain combined with the brighter BW sound is probably not the best match. You'd want a more laid back speaker or a more refined amp. McCormack, Pass, Classe as mentioned earlier are all great choices. High current would be best to get you the bass and resolution you'd want from your favorite recording artists you mention.

I agree that the preamp could also be part of the problem. So many of the early 90s preamps are aggressive/forward sounding, perhaps due to their design and due to decline of caps simply due to age.

I've had two Adcoms, the 535 and the 5500. I've also had two McCormack's, the DNA 0.5 and 1.0 monoblocks. The McCormacks are head and shoulders above the Adcoms in every way. I had severe listener fatigue problems with the Adcoms, which improved when I bought the fist McCormacks.
There is no doubt that will get you closer to where you want to be. Also, don't underestimate the 0.5's ability to drive demanding speakers. It did much bettery with my maggies (1.6QR's) than the 200 WPC Adcom did. Also, most of the unmodified amps have 100 kOhm input impedance, so any pre will drive them. I'd say go for it then see where you are.

Brownsfan makes a good point -- it might be the modded McCormacks that have a lower impedance input.

I recall from past experience enjoying the sound that the McCormack amp made driving a pair of 801 series II. Definitely less harsh than the Adcom. The 801's are very similar to your 803's in terms of its sonic signature. The 801's are known to be very picky about which amp to use. The Adcom GFA 555 II amps are very powerful but don't have the refinement and resolution that the B&W's demand.