the DT50 does not have preamp tubes...
it has an inverter 12ax7 like the dt25
and a boost 12ax7 which the dt25 does solid state...
all pre-amps are entirely modeled.
I sent my DT50 2X12 in for repairs. Here is how the support ticket went:
*Can you tell me what the hell was up with my DT?
Looks like the tubes were going bad. Normally there's just a 90-day warranty on the tubes, however we replaced the preamp and power tubes on your amp and rebiased them.
*Preamp tubes? What you talkin' 'bout Willis? There aren't any Preamp tubes. Right? or am I totally out to lunch?
*Hi, yes there are preamp tubes, they're the 12AX7's.
There seems to be a bit of a disconnect. This Line 6 person says that indeed, the DT50 has preamp tubes. You are saying that it doesn't.
I think a comparative diagram would be helpful. One of a "normal" tube amp and one of a DT amp.