let me try to summarize:
I played in a tube amp, it was working great and all. The next day, I played again (but only for a very short ammount of time), and it was sounding terrible - it had this extreme buzzy/thin high frequency, as if you do a really bad quality recording of a guitar with LOADS of distortion, you know? Unfortunately I don't have any recordings of the sound to show exactly what it is, nor do I have access to the amp right now.

Anyways, the only way I found out to get rid of it was to put mids and highs on zero, presence pretty low (like at most 2 - 3), and very low gain (about 1). This is a bugera amp btw, one of those I believe: http://www.bugera-amps.com/images/products/6262-212_P0669_Front_web.png (6262-212) though I have no way of confirming if that's the exact model or whatever, but it does look exactly like it so I guess it is.

I was using the lead channel, though it was also presenting this bad sound in the other channels (clean / crunch).

I am guessing it's a tube gone bad? any ideas of how to trouble shoot it? I have no access to it right now, but in a few days I'll be able to.
Almost certainly a tube. Buy some replacements, swap one at a time until the trouble goes away.
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
well the easiest way is to have a spare preamp and power tube...which you should already own. take 1 new tube and start swapping it for each preamp tube and seeing if that fixes an issue. same goes for power tubes.

relatively speaking, tubes are cheap and you pretty much need em if one goes out so why not stock a spare or two? its not like they perish in the box.

secondly, bugera preamp tubes are generally Sovtek 12ax7wa. those are GARBAGE. if you have those, please swap them out. the tonal improvement will be noticable. i found bugeras with these preamps to be very buzzy. i really enjoyed a 5751 tube in the first position. it really added a lot of clarity and punch and got rid of the box of bees.

how old is the amp? i mean, tubes can go out at any time, but if you have been using it for a while, a tube change may be beneficial. stock tubes are a risk. they are low quality generally, and have been bouncing around in warehouses, etc. chances are they fail for than quality aftermarket tubes.

replacing with tubes from a reputable dealer who tests them, get some name brand like JJ, tung sol, mullard, etc...get the matched pairs etc, they will most likely sounds slightly better and last longer than the stock pair, especially in an amp like bugera.

in almost every stock amp ive ever owned i had a power tube fail within the first year, bugera, engater, carvin, etc.
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