Received a set of the Sylvania VT-231's last week and have been burning them in. I had been using two CBS Hytrons as outputs and one EH as the gain tube. I think the Sylvanias are almost NOS as they seem to need burn in but burned into the system in only about 5 days. There was an overall improvement in everything with the three VT-231's, but not huge. I had a feeling that rolling was going to be in my immediate future again, but wanted to get a good idea of what the baseline all VT-231 "sound" was like. My first roll was to use two Hytrons as outputs and a VT-231 as the gain. Put'em in and the first thing I noticed was broken in tubes stay broken in; they don't lose it like solid state when power is absent. Cool! They just pick up where they left off. I let everything play and gel for about 4 hours out of superstition, anyway. Despite being in the output position the Hytrons had the most influence in this case. The overall sound went warmer and rounder, the soundstage became less layered, more recessed and less dynamic. The "Wall of Sound" quality of the amp (MPX3) is still there but in a different way. If you just tuned in you would think this is bad stuff, but it's not at all bad. What you get is the most amazing open midrange and highend. Super transparent and with big air and yes, you do still have Bass. The sound is light, ethereal, romantic and beautiful.
The other combination I've enjoyed since last night was to have the two outputs be the Sylvanias and a Hytron as the gain. This combination yielded the best synergy. The sound is equal to more than either tube. This is what I'm keeping in the MPX3 until I get ready to roll in a Ken Rad coming next week. This combo is the very best I've tried so far. Big soundstage, excellent "Wall of Sound", (I need to get some Phil Spector) layers and layers of imaging; very 3D images. Sound can be fast or slow, very good bass, mids, excellent highend detail and dynamics- better than using the VT's and Hytrons by themselves.
I was lucky to have Synergy with me this week.