Buy tubes from Audio Research or not

There are many tube companies out there like Newsensor. In fact that may be wear audio research is getting their tubes from. They own the Sovtek tube factory in Russia. No difference except Audio Research tests the tubes they get.

I just re-tubed my ARC D-70 Mk2 and I bought a matched quad set of winged C 6550's from Antique Electronic Supply. I also bought NOS GE 6FQ7's and JJ 6922 from them as well. I installed the tubes( probably easier to do than on a VT100mk111) and had my tech do the biasing. He discovered a bogus 6922 and so he replaced that at the time. ( I bought a couple of spare 6922's for this purpose)
I think the biggie with these amps is the biasing procedure, IMHO that is definitely NOT for the faint of heart.. ARC seems to think that the layman can do this procedure, to which I say they are NUTS!! Not only are you dealing with lethal voltages in the amp, but the various points to determine where to set the meter are not even marked, at least in my amp. ( and you need somespecial tools to do the job)
In regards to whether it is better to buy the tubes from ARC, i am of two minds on that... OTOH, I like to support ARC for their loyalty to their customers, OTOH, I think their pricing is not really reasonable and in my case
I wanted to tube roll, which they really don't do.
If you go with SED 6550's then I think that the odds are great that these tubes will work well in your amp and can be bought from any respectable supplier.

Buy "Perfect Matched" sets of Winged "C" (SED) 6550C tubes from The Tube Store (Canada).

They are just as good as ARC's EXPENSIVE sets.

I have used both in my ARC D125 power amp.

I second Don c55 here. Over the years, i have purchased tubes like KT91 & KT88 from ARC and 6550C Winged C from The Tube Store and Parts Connexion for my Classic 120 monos. Biasing is very straight forward (unlike other ARC models). One biasing pot per amp. The only problem i have had when i changed the KT91 to 6550C, i did not have enough range on the pot to adjust down to .65ma. I had to replace one of the resistor beside the pot. ARC sent me so resistors of different values at no charge. Another problem i have experienced about 5 years ago, the 6550c Winged C were produced with smaller pins and fit loosely in the sockets. That issue was wide spread worldwide and an outcry ensued. Fortunately, mine are mounted vertically but those that are mounted horizontally, many people had reported their tubes sagging from their sockets. ARC use much better sockets today.

And if ARC is like cj (which I assume they are), they aren't using tube testers to select their tubes. They are testing the tubes performance in circuit, esp. for noise. One would be surprised by the reject rate.