The two best KT88 single ended amps I've heard are:
Acoustic Masterpiece M101. This is an Air-Tight design using terrific Tamura transformers. It can be wired as an "SET" in psuedo-triode mode for 5/5w or normally for 8/8w. It is among the half dozen clearest and most objective amplifiers I've heard. It is not a good candidate for use with a passive preamp, however. It has just a 12ax7 ahead of the KT88, so voltage gain is low, input sensitivity is low. It takes over 2 volts to drive it to full power.
Audion makes excellent SE KT88 amps in a circuit they call "ETSE" or Extended Triode Single-Ended. Their single tube KT88 amps are 18/18w and have very high input sensitivity, so they are easily driven with small signals.
Both amps work well on 100 db/w/m speakers. I know as I have actual experience with such a combination.
I know that the US Audion importer has a close-out on some demo amps from their current line.
I am going to be posting my previous-generation Audion ETSE KT88 integrated amp (with remote and 4-input switching) for sale soon in your stated price range. And I am going to be posting an Acoustic Masterpiece M101 soon as well, which I've used to drive Stax electrostatic headphones. If you have any interest, you're welcome to inquire. Or I'm happy to refer you to the Audion importer.
A single-ended KT88 amp is an especially good choice as a first tube amp for someone who has been listening to transistors. They are generally fast and open, with firm, articulate bass compared to most SETs.
Phil
