I have a DSL 401 i've had for a couple years with no problems. Recently when I play for 10-15 minutes the sound immediately goes VERY quiet. The overdrive is still there its just very quiet (I think that means pre-amp is ok but power amp having issues?). The harder I push the amp the faster it does this to me and it won't fix itself until I turn it on for a while. I've used some extra fans blowing into the back and stuff to see if it was overheating and nothing seems to work. Also I just put in new tubes and it was having this problem and before and after the tube switch so I know its not the tubes...

Any ideas?
It could have been old ubes before and a dud now. But that's not really likely. Pull out the chassis and look for burned out caps, fuses, etc.
Did you actually replace the power tubes, or the preamp tubes?
If you did, do what mcrfobtai said and look for some fidgety components.
Call me Wes.
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^dark arbiter has come back!where the **** were you

sorry thtas my only contribution to this thread

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Yeh the SICK! bit sounds a bit stupid.

Yeah, it sounds like a tube problem to me... but we still need to know which tubes you replaced.

Replaced all 8... Pre-amp and Power-amp tubes... problem existed the same before... thats why I replaced them... 1. I'd had this a couple years without putting new tubes in and 2. it was having this issue. Tubes did not change it at all...
Have you checked all connections, guitar to amp and the speaker cables from amp to cab?

I suspect it's your amp's input jack if you've replaced all tubes and you're still having problems.
Cable could be the cause, too.
Also, do you use any pedals powered by batteries? B/c when the battery in my Bad Monkey dies, it kind of cuts off the signal...

Sounds like a resistor in the power supply has a bad solder joint. When you play for a while, it heats up and disconnects and your volume dies. Higher volume means more heat so it dies more quickly.
I'd pull the chassis and get it up on some blocks with a fan blowing into it, then see if it does the same thing. If it takes longer to die or doesn't at all, then you need to check your B+ supply resistors.
If you haven't pulled a chassis before, google safety procedures before you do anything just so you're aware of what's dangerous and what's not.