Hey everyone, my amp head (Peavey 6505+) has recently started sounding 'off' to my ear. I'm trying to figure out the issue myself before paying my only local tech $130 to troubleshoot it. The symptoms I've found are-

The amp sounds MUCH better at quiet volumes now (Below 2).

When the volume is turned anywhere above roughly 2 1/2, it sounds muddy and looses presence/treble, and continues to get worse as it's turned up.

It's definitely quieter at louder volumes than it used to be.

The amp, overall, sounds weaker and less punchy than it used to. The best word I can use to describe the sound is 'distant'; as if it were being played in another room.

I'm suspect of my 6L6's, but I just had them changed out about a year ago. What do you guys think?
Sounds like tubes to me.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
Sounds like the power tubes. Buy yourself a nice 12AX7/ECC83 as well while you're at it. They're cheap and you're already paying for freight. Then after you replace the power tubes, experiment with the new preamp tube in different places. You'll know pretty quick what's going on, just use your ears.
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Marshall 1960A
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Cathbard Amplification
My band
I don't know how hard you normally drive your amp, but what you describe just started happening to my amp after about nine months (my volume is always at about 1:30 with an occasional bump to 3:00 for some nastiness), although, I didn't lose any volume. I'm pretty quick to pick up on a high-end loss because I play with a pretty bright sound to begin with, so I switched to the other pair with plenty of life still left in them (I run in 40W mode just to have a spare set right there waiting for me).

As far as preamp tubes go, it's possible they can last 20 years so I doubt it's that. But, if you have the preamp tubes that came with the amp, I think you would be pleasantly surprised by an upgrade anyway. Personally, I use Tung-Sol re-issue 12AX7's (long plate-like bottom end, low noise, very well balanced, superb clarity, reliable). The upgrade from Mesa's stock tubes was like night and day. I hear new-old-stock tubes are really good but I haven't felt enough of a urge to drop the cash on any yet so I couldn't say.
"Practice doesn't make perfect. Perfect practice makes perfect." -some dude