FWIW, since you seem to be dedicated to buying a tubed pre-amp I will restrain my self from comments on most other things, and comment on that issue.
1) Be sure your selection is electrially matched to perform best with your amp, i.e. correct impedence values in both units.
2) Be sure that the unit you buy will give you long range satisfaction even if it proves not to be the solution to the problem you are presently experiencing. If it does, your in hog heven, if not you can go to the next possible solution, not backward.
If your budget allows, consider the Joule Electra LA100 III pre-amp. One is presently on the AG. (That is what I would do anyway.)
BTW, my most successful, initial departure from the classic upper midrange brightness issue was the purchase of a high end tubed pre-amp, and subsequently a tubed CDP, if for no other reason, that I could change tonal issues by just changing tubes. The next step was fine tuning set up issues, and very belatedly finding the right speakers (for me and my room) and the amplification for them. Took me many years and a lot of experiments. No quick fixes that I ever found.
A last, but long, comment. All of the recomendations about recordings being critical are right on point as I initially implied, however a couple of things to think about.
1) Most recordings are made with a prospective user in mind, i.e. deaf, dumb, and blind, a beginning audiophile with a mid-fi solid state system, an advanced audiophile (many who post here), and the SOTA folk who seem to listen mostly to sound effects, valuing such things as depth of image (specificity, transparency, resolution) VERY highly.
2) Consider, for example, that the recording recommended by Al, great as it probably is, may not help you tune your system. It will undoubted sound good, and unless you have heard it over a reference system, you really don't know what its true potential is. But, just taking those specific old recordings you have at hand, you can play them on your 'new' system and judge your progress in tuning out the upper mid-range brightness. My favorite reference disc is "Depth of Image" on OPUS 3. A simple miked recording of various music with a description of what one SHOULD hear on each cut. I heard this on a reference system. WOW! So it became my guide and helped immensely.
Good luck..........