In the Mod 50, TS 5881s sound the best to me with the VX.
In the M4/RT250 rig, I run GTE34L on Ch1 and Sovtek 5881 on Ch2 running the poweramp in stereo. This rig just kills with the VX, moreso than the Mod 50, regardless of tubes. I've run the VX with EL84, but to be honest, I didn't like the overall result. If you want the most vox-like sound out the VX, you need a 2x12 with the correct speakers.