I dunno, recently, there seems to be something wrong with my amp. I'm not sure of the model, but it's a pretty old Marshall 50w.

Anyways, the tone is suddenly really crappy, specifically, the sound is incredibly choppy when playing power chords and stuff. As in, the sound doesn't blend, instead it's like this really choppy sound that really sucks.

I play with the same settings but the sound is definitely different nowadays. Does anyone know what could be wrong with it?
sounds like fried tubes (valves)
Fernandes Revolver Elite
Schecter C1 Exotic
Schecter C1 Classic
Peavey JSX head
Hartke 4x12 cab
Boss GT-8
BBE 362 Sonic maximizer
if by pretty old you mean the old MG series, burn it.

if its an older marshall tube amp(valve amp) its probably fried/damaged tubes.

Epiphone Hummingbird
Epiphone Futura Custom Prophecy (Twin EMG)
Vox Valvetronix VT20+
Vox Wah
Boss MD-2
Danelectro Cool Cat Drive
Boss EQ

Make a picture if you can't tell us which model it is.
Fender American Special HSS Stratocaster
Ibanez 1987 Roadstar II Deluxe
Yamaha THR10X
Marshall JCM900 SL-X
Ibanez WD-7 Weeping Demon Wah
TC Electronic Polytune
Seymour Duncan Tweakfuzz