This may apply to other amps as well, but since I know fuck all about tubes other than my own, I'll throw this out there.
I was just talking to Dave, listening to him suffer from bubonic plague on the phone, and brought up the idea of swapping out the stock 12AX7 in V5 (FX send return) with a TungSol. In V1, the TS added some sparkle () and good top end everywhere, and got rid of some fizz.
What are you Mesa-heads thoughts on slapping one in V5 as well? The Roadster does bring up a bit more fizz and brightness with the loop engaged, and I'm thinking that the TS could "tame" that in a similar way to V1.
When I had a triple, I ran a patch cable from the FX Send to FX Return, and I like the bit of smoothing it added. Putting a nice sounding tube in the FX loop could help.