I'm having the same problem. Everything is fine just running strat with POD hd500 via l6link, playing along and then without any reason the sound suddenly goes dark, slightly tinny, and volume drops considerably. Oddly if I play some chords with lower bass tones or low notes it seems to kick it back into a normal sound......but sometimes it doesn't. It is very strange.
How do you load firmware to this speaker by the way?