KT88 vs 6550 tubes

My experience is that the vintage NOS tubes are considerably more reliable.
As for Ebay while I have bought successfully there, when it comes to high dollar output tubes I am hesitant. 
I wil say that the tubes you use in these amps can make a big difference.
Input and driver tubes can influence things greatly. Siemens-Halske 12AT7s are quite nice. RCA 12AX7s seem to round things out. Good bass. Andy at Vintage Tubes provided my AX7s. 

@dentdog thats true. I didn’t like the EAT tubes at all as they sounded muddy and lacked bass, despite being priced at $450 each. Popped my RCA 6550s and unit came back to life lol. I’ll see if I can track down a quad of black plates 

Try Tube Depot and see what they have.

https://www.tubedepot.com/

you probably got a bad tube. I would contact your seller for a replacement I have been running Golden Lion reissue and Shuguang 50 year treasures in my Mcintosh 275 vi for about 5 years with no issues whatsoever 

Yep... you got a bad tube.  I've got a number of BAD tubes from ebay sellers stating them as new.  (I suspect the tubes are factory seconds.) The tubes from certain ebay sellers measure poorer than the ARC supplied tubes with 2000+ hours on them.  I only now purchase from ARC, Tube Depot or Upscale audio.  I've had multiple EH 6992 tubes blow after 30 or 40 hours, from a seller on ebay; I inserted the original ARC Sovtek 6992's (with 1000's of hours on them 2 years ago.) and everything is great.