has anyone else experience this.....

How much time do you have on the Mullards? They need at least 40-50 hours to burn in.

I use Mullards almost exclusively and have never found what you state to be the norm. Maybe new, right out of the box, but never after they've burned in.

Give them some time to break in...

There is something wrong with the tubes, (they might just be
worn out? How used is used?) or your cj PV-10 just doesn't
play well with them or you just don't like them. Maybe time
to move on.

Sorry, but what you describe, isn't what I have found to be
true with NOS Mullards. I have been using these tubes for
over 30 years and still have bunches of them left. By far
"my" favorite tube, but definitely not for
everyone. As always, YMMV...