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.