Oddly enough, my most disasterous system setup was in response to suggestions for best quality sound. I had read on other forums about how "incredible" tube only Fisher gear sounds in a vinyl only setup. Well, I had a working Fisher 100-T preamp and freshly recapped SA-16 (considered one of the best sounding Fisher amps ever made) that I had used independently and decided to try. Separately, the preamp had a bit of a darker coloring and fuzzing up of the sound than I typically prefer (more so than my Marantz 3200), but it hadn't been objectionable. The SA-16 had sounded great with a solid state preamplifier and I had already been using it on my Flatlines. When I connected them both together and hooked up my vinyl rig, I was definitely incredulous. I've never heard such a thick, mushy, distorted sound in my listening system. It was the sonic equivalent of "molasses in winter." There were no transients, no highs, half-note bass (not even worthy of being called one-note), and it was difficult to tell instruments apart. What I had been led to believe would be sonic nirvana made a clock radio sound like a live performance.

I suppose folks would like the sound, but it was nothing like real music.
To state an overly bright combination as others have, I'd have to say my worst pairing there was a Kenwood KA-5500 and Pioneer HPM-100s. The former is a bit harsh/grainy on the top end and the latter peaks 12+dB in the treble itself. The two combined would work great to chase away in-laws and other pests.
- JP