Those GE's are going to be hard to beat!!! However, if you haven't already done so, check out the Audioasylum (www.audioasylum.com). Look in the tubes section and do a search for 6550. You'll get lots of information and lots of opinions. If you don't mind spending the money, it will be very difficult to find anything better sounding or more reliable than NOS Tung-sol or GE.
How to pick which brand of 6550 tubes to purchase
I have a pair of Ray Lumley M-100 amps. Have used two complete sets GE 6550 for close to twenty years. Need new tubes. Which tubes would you recommend to purchase? Any knowledge of the sound character of the different models. I like the accurate/neutral type of sound versus the too valve sound.