Hey Charlie, Yes all broken in and sounding excellent. Gold Lion KT88 are great sounding and great quality tubes. The matching was incredibly accurate so no problems. They biased up steady. After a month they rose about 10% so I need to turn them down a little. I'm a bit afraid to put my hand in the high voltage chassis, but it's pretty easy to do on mine.
Yes the bias is always different, even replacing the same type tubes from the same brand. You have to turn bias all the way down (usually unintuitively clockwise,) warm up the tubes and bring up the bias slowly to 90%, let it heat up more, then final adjustment. Then recheck after a day, a week and a couple months to note any drift. New Sensor family tubes are usually very steady bias, ime.
Retubing the 250 sounds like a fun project! NOS power tube prices are very high and they will not be matched as needed by AR. There are no more SED 6550C available, they were called Svetlana when yours were made, then NewSensor got the rights to 'Svetlana' so the real Svetlana rebranded as SED for American sales, now they are belly up, or at least the 6550s are all done. AR, VTL and others used the SEDs originally. But AR has switched completely to NewSensor for everything. The new production tubes from NewSensor are excellent quality and that's what AR sells for retubing, Sovtek brand. Same mfg as GoldLion, TungSol, Electro Harmonix, etc, American owned company in Russia. Many of your small tubes are probably still OK, test em. Make sure they are marked with Sharpie when you remove them from the amp as older AR amps had small tube bias adjusts. Definitely obtain written bias instructions from AR before bringing it to a local tech. If they try to do it without instructions kablammo. Some of the startup procedures of the older amps were pretty complex.
A quad of sovtek 6550 as you would get ordering direct from AR is $125/quad from McShane. He burns tests and matches with similar quality as AR, probably about half the price. TungSols are supposed to be "more refined" with excellent bass $140/qd, Gold Lions are "the best" for $220/qd. ymmv For that many tubes in an old amp, I'd do the Sovteks. Probably just as good as SEDs.
http://www.mcshanedesign.net/tubes.htm Wherever you get them must do thorough burn in and matching, not "factory matched" BS. Upscale is supposedly good at matching. I've bought from tubedepot a few years ago, they weren't as well matched as I would like. AR needs closely matched tubes. McShane is a tech, he understand what different amps need and consequences of mistake. He can offer you advice of what to put in there.
Found the manual online, has bias instructions:
http://www.audioresearch.com/ContentsFiles%5CD250IIServo_Manual_Schem.pdf