pre-amp unacceptable noise level?

Have you tried a cheater plug on your amp. Sounds like you might just have a ground loop.

If that doesn't work you might just need some low noise tubes.

What does the manufacturer say your problem might be? A very light hum or hiss heard from less than a foot from your speaker is not unusual IF you have very efficient speakers or an amp with a very high input sensitivity.

Which preamp did you get?

Humming sounds like a grounding problem. Is your TT grounded to the amp or pre-amp? You might try moving it around to different components.

Are all you components plugged into same outlet? If not, try that. If none of that works, then try the cheater plugs.

Dave