I suggest the OP could use a few more direct thoughts on what he experienced, which was that a switch in preamps provided engaging midrange but a lack of bass and soundstage...perhaps that means HF detail to pinpoint stereo image?
Keep in mind this may have nothing to do with tubes at all. Much of the sound quality of a component system comes from the relationship and synergy of the components.
This is nowhere more true than the match between preamp and amp. Impedance, how hot the signal is...very relevant stuff.
Additionally, if the OP always has loudness on with his preamp, then either (1) he isn't actually that fond of its sound or (2) he listens at low volumes.
My suggestion: try different preamps until you get an amp match (or I suppose vice versa - but I'm presuming the amp and speakers are a good match to eliminate variables).
For example - you use the Lumin to control volume...try that directly to the power amp. Not my jam -- I have found I strongly prefer an actual active preamp in my system -- but it is worth it to get a sense of what needs effort.
See if you can borrow a couple other preamps. Or do the math and find one that matches the power amp.
Finally, I have found that tube buffers are a bit more of a novelty than anything else - one thing to actual use tubes in the necessary circuitry, but a tube buffer really cannot do more than coloration. At least the ones I am aware of - perhaps some help with component matching or similar (in which case put it b/w the pre- and the power amp).
My bottom line is that being somewhat analytical about upgrade paths is going to help you get to your objectives faster than just pulling out the wallet or taking gear recommendations from posters with different systems, different rooms, and different ears.
To me, matching gear is at least equally important to some perceived "gear quality" based on price or similar factors. Secondarily, people are constantly surprised at how toe-in (or not) on speakers, and adjusting front and rear feet on them, can make a massive difference.