ECC 91 / 6J6A good audio Quality?

I recently purchased a Vincent SP331 MK Amp.
It was advertised in used 12AU7 tube.
BUT!! After a 100hrs burn in. I open it
to see how well Mfg. What! NO 12AU7..
But a 6J6A/ ECC91 7 pin. Is there something
we all have been SONICLY missing about ECC99's
Or is the Mfg going on CHEAP??
I like to ROLL tubes. But 6j6A NOS go for $2 buck's+.