Can a bass-shy / polite tubed preamp be remedied?

Send it to Steve Huntley at Great Northern Sound

To answer tube pre modification part,
adding an inverting unity differential stage may do the trick. You can try it using Musical Fidelity X10 tube buffer which is realy what I mean above.
Changing an amplifier for the higher input impedance(I can only assume you have solid state amp) may also fix the problem.
If you live close-by, I can lend you my X10 so you can see if the problem is fixed. There are also cheap differential inverters(in $40-ish range) could be purchased at the pro music/audio stores so you can try it, return it back and shop for the product of a higher quality or mod your preamp.

Marakanetz, The Musical Fidelity X10 tube buffer sounds interesting. Do you live in the NY area?

Yes I do, in Brooklyn.