Jolida 502A Popping Noise in Left Channel

Send the following text to Michael Allen of Jolida this morning, and figured I would post it here to get some suggestions, thoughts and solutions from y'all while waiting for Michael's reply. Btw, I"ve "XXXXX" out the name of company doing the work, as there may be solution to this problem they can correct, and wouldn't want to hurt a company's reputation unnecessarly.

"Got a question on a "noise problem" I'm having with my Jolida 502A amp. First of all, thanks for sending me the "fuseholder". It was different than the orginal one (actually it was of "better quaility" - and I certainly appreciated that it had a screw top), so I called XXXXX and inquired how much it would cost to have the fuseholder installed. Their cost was $75 because of a one hour labor rate. Well, I figured if they were going to be "under the hood", so to speak and that the amp itself is ten years old that it would be a good time to check the amp out and replaces some caps and resistors and basically give the amp a "tune up". Anyway, got the amp back and placed it back into my system and turned it on to let the tubes warm up. I would say at approximately 30 seconds to 1 minute after turning the amp on, I hear a "squealing" sound out of my left speaker, followed by "humming" sound in the left speaker. After hearing that, I disconnected and returned it to XXXXX, to find out what was causing the problem. Anyway, the first thing they did was check all the tubes where it discoverd that three of my pre-amp tubes were shot, and at this time they also checked work done on the amp itself, and didn't find any problems there. At that point, the pre-amp tubes were replaced with some Jolida tubes (for testing purposes), and the amp was played to see if the "hum" problem was solved. And for awhile, during this test, it seem to be so, until the bottom cover of the amp was put back into place and the "hum" re-appeared in the left channel. At this point, the Power tubes (Tung-Sol 6550) were checked to see if one of them might be causing the problem and sure enough, it appeared that the tube in the V-8 position couldn't/wouldn't hold it's bias, so that tube was replaced and the amp was re-biased and the "hum" problem disappeared. Took the amp home, re-installed it in my system, and placed some older pre-amp tubes (Sylvania 12AT7, Sovtex 12AX7) into the amp (did this as I wanted to make sure that the amp was working properly before installing the RCA 12AT7, Tung-Sol 12AX7 that I had just purchased), turned the amp on, and after the tubes had warmed up, proceed to play music, and everything was sounding fine. Indeed it stayed this was throughout the weekend where I put probably 10 - 12 hours playing music through the amp. Well Monday, I figured the amp was indeed working well, and there didn't seem to be any problems, so I figured I would go ahead and replace the Sylvania/Sovtex pre-amp tubes with the RCA/Tung-Sol tubes (as I prefer the sound of this combination). Imagine my surprise, when shortly after turning the amp on that I hear some intermittment "popping" noise through the left speaker. Now these noise subsided after about 5 minutes and the amp was quiet (no "humming', ect) and when I played some music throught the amp, once again everything sounded fine. I did noticed that when I turned the amp off, there was a "thump" sound in the left speakers, which I had never heard before. Yesterday, I figured I turn the amp on, and see if the things that happened on Monday, happen again. And things went pretty much the same as they did on Monday. The one thing I did notice yesterday that when I first turn the amp on and was letting the tubes warm up, was that at one point the power tube in the V-8 position, light seem to "flutter" for a couple of seconds, and then stablized. btw, this was the tube that was re-placed at XXXXX. Anyway, I know I've gone on and on, in my description, but figured the more information you have the better able you'll be able to tell me what's going on, and what would might be the solution to solve the problem with the "popping" sound I hear in the left channel when turning the amp on and off. Other than that I'm happy with work done on the amp, as I feel that replacing the caps and resistors certainly upgraded the performance of the amp, in all area."