I have the 502B and 100 myself.
I have tried various current production tubes.
I have tried EH, EI, Groove Tubes and the Sovtek LPS (and a couple others).
I settled on the Sovteks before hearing the Groove tubes,
then switched to the Groove tubes but after a couple months went back to the Sovteks in both units.
Both are good but the Sovteks are more musical to me.
I have not tried any NOS.
FWIW, I like the Genalex Gold Lion KT-88 reissues the best for the power tubes.
Enjoy,,,,
I have tried various current production tubes.
I have tried EH, EI, Groove Tubes and the Sovtek LPS (and a couple others).
I settled on the Sovteks before hearing the Groove tubes,
then switched to the Groove tubes but after a couple months went back to the Sovteks in both units.
Both are good but the Sovteks are more musical to me.
I have not tried any NOS.
FWIW, I like the Genalex Gold Lion KT-88 reissues the best for the power tubes.
Enjoy,,,,