I was playing through my 6505+ the other day, and after a few minutes of some sporadic, subtle oddities response-wise, the sound just died out of nowhere. All the lights still come on, all the tubes are still lit up yellow with just a hint of blue (they're about a year old, but I haven't played on the amp a whole lot during that time); just no sound. I've tried different guitars, with and without front effects, with and without loop, with and without channel switch. The only way I can get a different result is kicking in my Tube Screamer without my noise gate on, in which case I get some faint hissing. My first thought was that I lost some tubes, and while I don't know for sure since I'm fairly new to tube amps, I thought losing a tube would make more noise than, well, just suddenly none. I wanted to see if anyone could identify an obvious problem based on my description before I just went in and started ripping stuff out. Any input would be much appreciated.
I'm assuming it's quiet on both channels?

Try plugging your guitar directly into the effects loop "send" jack. If there's sound, it will be clean. If so, that means your power tubes and power section are probably okay.

It's probably a bad preamp tube, in all likelihood. And probably V1 (the one closest to the input jack of the amp), since you didn't mention it making sound on either channel. V1 is kind of the "master" preamp tube position in that it's the first tube in the chain and powers both channels. If that one is bad, you wouldn't get any sound out of the amp unless you plugged directly into the power section as I mentioned above.

There are other preamp positions in which you could have sound on one channel but not the other -- V2, I believe. That happened to me.

The positions of your preamp tubes, when looking from the BACK of the amp with the panel off, from left to right are V1, V2, V3, V4. V5, which is your phase inverter, is the one that is not aligned with the others and is tucked back close to your reverb tank. If that one were bad, you would also not get any sound out of the amp on either channel.

My suggestion is to buy a new 12AX7 preamp tube (preferably a JJ since they sound so good in 6505s). Then just go through your preamp positions trying it out. If you're lucky, it's just V1 and you'll have everything running again for around $12-13.
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Yes, it's quiet on both channels. I'll try that, thanks for the help!