VTL TL2.5---factory installed tubes?

Hello---I recently purchased a used VTL TL2.5 pre here on Audiogon and was wondering if anyone knows what tubes VTL originally installed in this model at the factory.

The seller included several pairs of various brand 12au7 tubes (Tung Sol, Tesla, Phillips), but I don't really know which ones "belong" in there. The Tung Sol tubes were in there when I got it, but those turned out to be way too microphonic, as well as giving off a high-pitched hum. I've got the Teslas in there now, they sound better than the Phillips to my ears. But I have the feeling this preamp isn't reaching it's full potential with these tubes, and I figure a good place to start would be to get the original VTL approved, factory-installed ones.

I e-mailed VTL about wanting to buy new stock tubes from them, but never got a response. Called and left a message, and they never returned my call. What gives?

I'm new to tubes, so any advice would be appreciated. Thanks, Will Herrera