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.

Gthirteen,
CJ and Rogue you've mentioned are fall onto the category of hobbyist-designed and built products. For its performance and reliability they should realy cost much less than being currently sold.

It's realy new for me to se CAT for less than a kilobuck but if that's the case than it's realy a great deal.

I always was excited the way Audio Research designs any product they sell and agree that their models deserve attention for all.

You can take my kilobuck statement as in-out range and in on more or less degree absolutely subjectively on inexpencive products something is missing what should be present.

Basically all I'm saying is not to fall onto belief that it will improve cause very often it will not if not carefully examined and/or tested.
For example, quite objectively, if I look onto Foreplay that is sold with kits or assembled by some small hobbyist company for quite cheap why should I believe that it will sound better than SS preamp manufactured by large manufacturer such as McIntosh, Audio Research or Plinius?