I started getting an unwanted distortion from my 68 CVR at a gig.. No pops or clicks, just a mushy lack of bass kind of sound that stuck around for the last few numbers. I had some old preamp tubes in there so I replaced them all. Still mushy. Changed out the two 6L6s still no good. At bedroom volumes it sounds fine, but at anything approaching 5 on the vintage channel, which is where I usually gig with it at, the mush kicks in. I noticed one of the tubes has some blue inside. Not something I haven't had in other 6L6 amps I have owned, which run fine like that, but this amp has never done that. Put the original 6L6 pair in again and the tube in the same socket is blueing up (never used to). Tried another set and same thing. Whichever tube I put in that socket goes blue. Is it worth me giving the socket a good cleaning with deoxit and or some contact cleaner? Or do I just go straight to the tech? Just for some clarity the tubes I'm putting in are matched pairs although I am NOT biasing them and I don't have any bias probes. I got it biased by a tech when I bought it and had to replace the original set of power tubes. The other duets I am putting in are exactly the same (although not rebiased). I've had no problems like this for 3-4 years with the amp and am generally very pleased with it. It was not biased hot by the tech as I asked for it that way, and have been delighted with the tone and headroom I get once I changed out the original speakers. I really don't want to take it in for repair but looks like I will have to. Just wondering if anyone has had a similar issue or can point me in the direction of diagnosing and repairing it myself, keeping in mind my knowledge is limited to 'external' fixes. I know what tubes do and how to replace them and I am able to slide out the chasis and look for obvious signs of damage, but I'm not about to test, diagnose or go about replacing a cap or resistor. In a nutshell: it sounds similar to a pi or power tube failing, but I know the tubes are good.