I have a little tube amp that has two Mullard (Webcor Brand) 6bqs and a
Telefunken 12AT7. I thought it would be fun to replace these tubes and hear
if I can tell a huge difference in sound. I was wondering what 12xxx to get
and how it would change the sound/bias of the amp.

Also, what are you opinion on the following tube brands for my amp:

Electro-Harmonix
Ei
JAN
JJ/Telsa
SINO
Sovetek

If you had to pick one brand above - what would it be?