My understanding of tubes in relationship to your problems: 1) As tubes get older, you will have to increase the bias voltage/amperage. I always turn DOWN the bias setting before installing new tubes(should need less bias for new tubes). I am using Atma-sphere M-60 Mk 2.2 monoblocks. I noticed that bias amperage will increase SLIGHTLY after about 1/2 to 3/4 hours of use. 2) I just had a weird and frustrating problem with a pair of 12AU7's (Telefunkens, no less!) After a few months, I was getting weird noises from the left channel, and the DC offset light kept illuminating on my Atma-sphere MP-3 preamp. Not a bad tube, but in working on some of the components inside the pre-amp, I must have bumped a tube, moving the pins slightly out of alignment in the socket holes. Also, your pins on the 12AT7's might be either slightly oxidized or UNDERSIZED for the socket holes. Clean the pins with Caig De-oxit, then use their G5x preservative. Use the G5x very sparingly on the pins, and YOU MUST USE THE G5x (extended temp.), NOT THE REGULAR G5!! You can check the pin sockets, bending slightly inward any that seem to have a larger gap. BE SURE TO LEAVE YOUR AMP/PREAMP OFF FOR AWHILE (I usually leave it UNPLUGGED overnight!) DO NOT TOUCH ANYTHING ELSE THAT IS METAL while working on the pin sockets!!! Using a small jewelers screw driver is your best bet. Also, there are many variants of ALL tubes, usually followed by a letter or letters. These can refer to the MAX. voltages in the tubes sections, and can vary! Check with the manufacturer, or Andy at Vintage Tube Services for specific applications:
http://www.vintagetubeservices.com/ (Tel # 616-454-3467)
Stay away from the other big name tube guru! The N.O.S 6AS7 power triode that one of his "helpers" recommended WOULD HAVE FAILED IMMEDIATELY! This "chowder-head" said that GAS7G's (I'm not sure of the last letter) were a direct replacement for GAS7's....NOT!!!! Andy is a tube whiz, the "real deal". As for buying on ebay, sometimes a bargain can turn out to be really expensive. You probably have a better chance pulling the handle of a slot machine!