300b tubes burn in time?

I'm sure you've already asked yourself this question but do you recall if your old ones sounded great right out of the box or if they too blossomed with decent burn in time?

Maybe it wasn't your output tubes that needed replaced?  Have you checked the gain or other tubes?  A worn out 12 series will make the 300b sound flat.  These shouldn't need burn in at all.
Also, have you checked your bias?

I bought the amps used, so the old ones sounded great all throughout (they must have had a few hundred hours on them at least I figure).

Same brand and model of tubes. Black treasures.

The other tubes should be fine (I think), since the old tubes gave great sound (I didn't replace them, its just one that I mishandled the amp and one of them broke) so am replacing not because the old tubes seem worn out, so I know there is still good sound if I still had the old tubes.

My amps are supposed to be auto-biased.

It's just not sounding that open or lively compared to what I am used to with this amp.