What I did was to build burn in circuits. I wired 4 sockets with 100K plate resistors and 1K cathode resistors and 1meg or what ever you have grid to ground. B+ might have been 140VDC not critical. I ran the heaters at 6.3VDC and let the tubes burn in for many hours I then switched to 5.7VDC for hours. I then tested them in circuit. Steve Benches site had a good write up on cycling tube heater voltages and tube noise. This seemed to help.
I tried Mullards GEs and other brand tubes.