Similar to what I feel about the impedance switch - unfortunately to me, as the sound getting thicker, the air/ss seems to narrow down, so I just stick with the L setting.
I'm not so sure about the feedback part though - just heard that both feedback and no feedback has their own drawnback, and one isn't exactly superior to another. Some of the very best amp I've heard have neg feedback (manley 300b/stingray/copland), while a few stuff which boosts no feedback anywhere (*cough* audio-gd *cough*) isn't that special.
You're right about the caps, the stock coupling caps are two tiny poly caps (SCR poly cap, not exactly the best for this type of duty). Still pondering on which cap to replace though. Generally I like Jensen, but they tend to sound a bit "crowded" and might not be the best match with the Icon. Once I decided on a cap I'll report back (will be either Mundorf S/G/O or Jensen, or even Duelund)