You might try replacing the preamp tube with a JJ Electronics tube. I have a set Im going to install into my TSL100 head when I have a spare moment.
You could also try talking to Bob at www.Eurotubes.com (US suppliers of the JJs and see what he has to say. He is a very knowledgeable fellow and can probably give you some good advice.
My instructors page and www.studiotrax.net for all things recording.
my toons Brought to you by Dr BadGAS
my toons Brought to you by Dr BadGAS