I also have two Mullard tubes $$$ for the preamp. As well as a remote. Never did put in the Mullards as the amp was under my monitor and life was busy. Who knows, maybe they would radically upgrade the amp. I think they would be significant as what's in there now is a cheap china tube. I also liked the sound so much that I was never bothered enough to change it out.
This really is a very nice sounding amp, esp. for the cost.
Rocket_Ronny