I'd say it's the easiest and cheapest way to experiment (these caps don't have a really high voltage rating). A little harder is coupling caps (voltage!), harder still is making global feedback frequency dependant. I think you might be surprised how low a value you need. I'd expect a PA amp to be more or less fullrange, if it's really dark there *might* be something off, depending on the rest of your rig, taste, etc.