Who makes the least expensive Tube Pre-Amp?

You will certainly hear ehem... difference especially after a while you'll start to develop sibliance and many unwanted colors.
Designing good tube pre is a complicated and expencive issue that figures at least $1000 for used unit.
You can search among vintage Audio Research or Macintosh tube preamp units otherwise not sure if any other small brands might produce something descent.

AES & Antique Sound labs.

watch for a used conrad johnson PV 5,6,or7 (300-500)they will perform with the ability to determine what your seeking. if you decided to not keep around, will be able to liquidate without any dificulty.

AMC makes cheap and decent tube pre.
OTherwise, please look into Bel Canto.
Good luck