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.