Hmm. So now we have the claim that:
(a) to hear what a preamp sounds like, try it with many amps and see what qualities persist through the changes.
vs.
(b) to hear what a preamp sounds like, keep the amp the same and then change between preamps to listen for the changes between preamps.
I suppose these are not necessarily at odds, though. They could be complementary?
Another option might also include
(c) Keep the preamp and amp the same, and change the tubes in the preamp only. See what qualities persist through tube changes.
I'd guess that (c) offers the least information, but it would offer some, no?
(a) to hear what a preamp sounds like, try it with many amps and see what qualities persist through the changes.
vs.
(b) to hear what a preamp sounds like, keep the amp the same and then change between preamps to listen for the changes between preamps.
I suppose these are not necessarily at odds, though. They could be complementary?
Another option might also include
(c) Keep the preamp and amp the same, and change the tubes in the preamp only. See what qualities persist through tube changes.
I'd guess that (c) offers the least information, but it would offer some, no?