Thanks for the tip, it reminds me of another point of conflict among V4 users. There seems to be a huge debate over weather you can swap 7027s for 6L6s, and I thought you couldn't, "because V4s run at much higher voltages than a 6L6 was designed to withstand" although I've heard that people do use 6L6s with no issues and it looks like the 7581A is just a 6L6 with more headroom. It doesn't help that TubeDepot put a warning under all their 7027s saying that they're not a direct replacement for 6L6
Alright, so this is a topic for someone with more knowledge on technical stuff like @electronicaudioexperiments or @crusherdestroyer because those are my V4 guys.
As it stands, with some tweaking you can put 6L6s in there but not just drop in. Hell, John played with 6550s in one as well if I remember correctly.
I think generally the 7581As are an easy move and they last a great amount of time and sound solid af. So that’s why they were rollin’ with them.