Well, my best explanation is that the difference is in the background presented by these products. We tend to listen to the music, just as when looking at a painting we look at the image. But in reality, a traditional painter has put real effort into selecting canvas or board, then covering it with gesso, then drawing in charcoal, and then painting a verdaccio version. Then what we see as the paint goes on. My point is that we do perceive the background (the underpainting, etc., that the old masters worked so assiduously to perfect). It’s just that we don’t see the background explicitly, but rather experience its effect on a more subliminal level.
I’d say that preamps have backgrounds, too. Describing them is difficult, but the Audio Research has a background that makes it seem slightly more airy and rich than the mbl or Marantz or the Parasound Halo. These background differences matter a lot to some listeners (myself included), but are precisely the kind of “differences” that are so subtle that, for some listeners, they seem either meaningless or non-existent. Subtlety isn’t automatically meaningless (perhaps the opposite), but by definition it isn’t obvious.
Two (or four) preamps don’t tell us everything we need to know to answer my opening questions. But I would say that the mid-priced preamp market clearly has some surprisingly good offerings. You can certainly get a multichannel preamp that performs very well on music while being affordable in high-end terms. It doesn’t seem likely to me that most people would need to go beyond this level of preamp. Preamps do differ meaningfully, though, so once you have everything dialed-in as well as you can, the issue of sonic backgrounds can become very important.