+1 Karl - good to see you are still around after all this time!
Line 6 reccomends using matched tubes and having them biased by a tech "in the know". I have performed a number of replacement/biases on the DTs and found it pretty straight forward in comparision to amps I have worked on in the past. If you have biased an amp it is pretty easy (25mv is reccomended for the DT25, 36v for the DT50). Also be sure the amp is set to Pentode and "class A/B" mode, not in "standby" mode - fell for that once or twice ;)
As the bias point pins are pretty close together I often use a single header cable to go on the negative point, then use an alligator clip to connect to the meter - it is a little too easy (for me) to touch the pos/neg together and create a "0.000" reading on the meter.
If you haven't biased an amp PLEASE take all necessary safety precautions, as there is potential for injury (or worse) if not performed correctly.