Hiwatt breakup and tube selection

[echo44]

so i scored a 50watt combo 1979 hiwatt with partrige transformers

has a real warm sound with marshellesque type breakup.
So i took it apart today and everything is original including old mullard
pre amp tube probably original. The EL34 not original are GE. so I tested the tubes and one tested "low good" and one tested "low worn" this is on the
new Orange digital tube tester. They look very worn with the mica base chipping off in places.

So I replaced them with NOS matched pair mullard xf2 which test
high good.

I than began the tube rolling and listening test with a friend. We both noticed the Mullard sounded very harsh? Brighter? than the failing tubes? I thought the mullard tubes should be a perfect match for this amp. Of interest with the mallards there definitely was a vox type sound coming from the amp and with the failing GE tubes more of a marshall smoother breakup. I than tried a pair of old Telefunken NOS which sounded somewhere in between.

would love to hear any opinions or advice. Didn't this amp probably originally ship with Mullards? Will the mullards smooth out with play as they get weaker? Any other tube recommendations?

[Roe]

hiwatts were designed around mullards. other preamp tubes often sound too bright

[old tube]

Mullard XF2's should sound smooth and clear, with a lot of detail coming through even at breakup.

Did you re-bias the amp? They could be running very hot or cold depending on how the amp was set around the old tubes.