I hope someone can answer if there are some serious mods to be done to upgrade this amp further.....besides those outlined by Cary.
YES- Changing the amp's driver tubes will have a major impact,
on it's presentation. Bottom-gettered Sylvanias(VT-231/JAN-
CHS) & Ken-Rads(VT-231/JAN-CKR), from the 40's, both offer
nice sound stage, depth and clarity. At least as much
noticeable difference as changing the input tube. The effect
is cumulative. Try to get a NOS pair that are matched, triode
to triode, as well as to one another.