Best 6550 NOS Power Tube for CJ Premier 11a?

new production 6550 kt88 very very good now... not sure NOS is the ticket unless you like to burn extra money

check out kevin deal or jim mcshane sites

to offer more exacting advice, i would need to know your intended amp, speakers and what you are looking for sound wise

there are now modern russian and chinese 6550/kt88 variants that deliver substantially more power and better bass response that old school/old design ones - kt90/kt120/kt150

using these tubes has been the whole model progression strategy for audio research tube amps for the last 10-15 years (since the vt series amps) -- objective being to deliver more clarity more solid and controlled bass more linearity, all the while preserving the 'tube magic' higher in the freq spectrum, so to speak

problem is not all amps can run with the higher bias currents and run these tubes properly 

so in your case you shd call jeff fischel at cj - ask him specifically if the prem 11a can handle higher power rated tube equivalents mentioned above... i suspect it may not, given its age

btw, the cj tube amps were quite musical sounding but definitely wimpy in the bass department... that’s why some 20 years ago i sold my prem 11a xs, went to arc, and never looked back - just one data point for you...