This is one of those questions with two extreme answers, and judging from what you've been saying, I think I know with one applies here.
If you want to dig into rebuilding the LUMI you can make it sound extremely good, clearing up the dark sound, etc. Or you can give up and buy something else. There are a few middle steps worth taking, but only rebuilding will really turn this into a less dark–and cleaner, I think--unit.
Before starting, I'm not sure what design you have. Mine was an older one, with a single HT supply, not the twin supplies that followed it. I suspect that the later ones had better components, though I'd have swapped them out, too.
You already heard from Kevin, a good friend and a wonderful designer in my experience. Much of what I did to the LUMI was at his suggestion. That includes his HT power supply and solid state regulators for the filaments, and a regulator for biasing the 6EM7. Each made a major improvement, especially the filament regulators. Scott Frankland's suggestions eliminated a coupling capacitor in each stage (a change that also reduced feedback and allowed me to exchange the 6DN7 for a 6SN7 type tube) and reduced some circuitry in the RIAA circuit. (The elimination of coupling capacitors may have been part of the designs after mine. If you've been talking to Scott, you probably know about his changes).
Beyond that, I can suggest the following.
Use 5691s instead of 6SL7. Mine came with 5691s; perhaps yours did. Believe it or not, the Electro-Harmonix 6SN7s work as well or better than 5692s. Use Sylvania 6EM7.
A huge component upgrade is required. Coupling caps have to be upgraded. I've gone through MITs and now use the new Teflon Vcaps. The latter was a major–and expensive–improvement. The big coupling caps remain Relcaps. All decoupling caps were replaced with Panasonics.
Resistors include Mills wirewounds, Kawames, metal films. Current requirements play a major role in resistor selection.
I went to stepped volume controls. Big difference.
Wire was a big problem in the unit I had. I ripped it all out and replaced all the power wire, umbilical, power cord, and signal wire with stuff I got from Michael Percy. I also found ways to eliminate a great deal of signal wire by rerouting parts of the circuit, etc. Again, major improvements.
I rebuilt the fusing circuits to run on one fuse rather than 2 or 3. (One fuse is eliminated right out when you built the filament regulators.)
Finally, I rebuilt all the grounds and added RFI blocking caps.
There's probably more, but that's what I recall.
