steve, there seems to be a trade-off w/the toob in the circuit - you're correct that it draws power from the p/s, but it also reduces elec'l leakage (i'm sure this isn't the correct technical term!) from the adc side into the dac. best bet is to try for yourself.. some prefer it one way, some the other. if a di/o's only mod is the upgraded p/s, & internal parts haven't been replaced, i'd be inclined to leave the tube in. if lotsa mods have been done, yank the tube...
doug s.