The VTF-3 also has the switch which changes the EQ and filtering to match a one-port configuration, so you get flatter response. I think it nudges the subsonic filter cutoff down a bit as well.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8