Inexplicable muddy sound on phono input

I've got a Rega turntable running into a Rowland Capri preamp. A few months after I purchased the turntable it developed a thick, wooly sound. I A-B'ed the same recording from vinyl and digital and the digital copy sounded great. So I changed the needle, but no luck. Then I took the turntable back to In Living Stereo here in NYC, and they could find nothing wrong, it sounded fine. Thinking it must be the phono pre, I shipped the preamp back to Sound and Vision, where I purchased it, and he found nothing wrong with it on that end. Yet the problem persists. I'm at a loss--I think my trouble shooting has covered every stage in the signal chain. Any suggestions? Thanks!