I suppose that you could do it from the front, but you'd need to be careful. Isopropyl alcohol can soften paint, so it might damage the fascia or the lettering.
If you want to try it, stand the amp on it's back, remove the knob, and trickle some isopropyl down the shaft. With a bit of luck, some will find its way inside. If the pot is a dual gang jobbie, getting isopropyl into the rearmost can might be tricky.
I'd remove the cover, and give the pot a good soaking.