Strategy 500 just outta storage, almost no volume

Hello. I've just taken my Strategy 500 out of being in storage for maybe 2 - 3 years, and have been checking it up, seeing how it's doing. When I put it into storage, it worked and sounded normal, apart from volume only recently starting to fade in and out on one side of the amp.

But now, after letting it warm up, then checking out the individual sides just now, both sides are almost entirely without volume, but if I crank the volume on either side, and max the output volume of my preamp with a guitar plugged in, a faint, distant sound can be heard, and what's heard sounds normal, just ultra quiet. The output wattage on the Strategy 500's chassis front meters seem to be showing an output range of all the way up to 200 watts with half-drive off, and up to 25 watts with half-drive mode mode on.

I tried swapping the 4 12ax7 preamp tubes with new, working ones, though that didn't change the situation, and I also tried swapping the 6550s with new KT88s, and that also didn't change the situation. I haven't tried swapping the 6L6s, as I don't have spares at the moment - though as I said the amp was at full volume when I put it into storage (with one side newly experiencing intermittent fade in and out).

Does someone here have a clearer perspective on what may be, or likely is behind the problem?

Here's a large picture I just took of the inside: http://i.imgur.com/W8qTN95.jpg

Also, I looked at the tubes while they were running, and none were glowing in any abnormal way. I'll take a picture of them, too: http://i.imgur.com/Slb10ot.jpg

I've got it worked out. I replaced all the 12ax7 and 6550 tubes together, and got one side of the amp playing normal. Maybe I just didn't test both sides after swapping one of the tube types before. I assume there was a bad 6550 on one side, and a bad 6L6 on the other - now there's just the bad 6L6 on the side that's still not normal. I'm going to swap things until I find out for sure, then keep the OK tubes for spares.

This thing sound incredible, I love it!