. . . to relate my own limited experience, I got into separates in '94 and it was an immediate entrance down the vacuum tube rabbit hole. Except for the preamp. Anyway, since then, I have only owned three different tube amps (I still own two of them) and 3 different preamps (and I still own all three of them), so I am not trying to come off as the know-all/be-all last word of wisdom.
My first upgrade (it was an amp ugrade) was a major equipment upgrade. Definite expansion of width and depth and height of soundstage . . . but still sort of missing something that I thought/knew must be out there. At that time I was using a B&K digital HT (Dolby Prologic) pre. It didn't sound bad, it didn't sound good, it just sounded. I came across a real affordable deal on a VAC two channel preamp with a pair of 12AX7s from Audio Advisor so I ordered it and I truly WANTED it to sound good. It didn't. Nails-on-a-chalkboard. I sent it back. In '99 I came across one of the few local dealers (he no longer exists as such) who had a used and wierdly modded Cary SLP90 for sale so I auditioned it for a weekend. It had one of the six tubes basically out, and the cabling the dealer sent me home with (an adapter to go from RCA out to balanced in on the pre) was totally wrong, but despite all that, and the fact that all of the above made it sound like hammered crap, it truly sounded musical. Based on the changes I felt I was going to have make, I made the dealer an offer that he accepted. I replaced the tubes and got the right cable (balanced out to balanced in) installed, and it was really the great sound I had wanted. (I later sent it to Cary to undo some of the previous mods.)
Down the road I changed amps again, not because of sonic quality but because of reliability issues, and about three years ago I saw what I thought was a decent price on a preowned preamp that I had always sort of been lusting for (plus the fact that I thought my SLP 90 was sounding tired), and the most recent new to me preamp I put in my system did impart sonic differences--not as sweet and warm as the preamp it replaced in my system, but more detailed and more "spacious" (if I dare use that word). A larger sonic presentation.
Anyway, TMI I am sure, but this is what I have personally experienced with different preamps and amps in my system.