So i have a Hot Rod Deluxe. It was 3 years old when i bought it used.
The powertubes were stock but the preamps had been replaced like a year ago.

The clean channel is great.. and stays great until it starts to get that powertube distortion at around 4.
And here is the problem. Instead of being a smooth overdrive, it is extremely choppy.

If it were powertubes, shouldn't it also affect the cleans?
The amps internal Drive channel is smooth at low volumes, but choppy at higher volumes.

What could be the malfunction?