I had an original Q-Tron (Electro-Harmonix) which did a fine job for most cover tunes where I used it. Sounded like everything I heard on old albums/CD's, etc and was very quiet, even when using the foot button.
I (regretfully) upgraded to a newer model Q-Tron that had a smaller footprint for my pedal board. E-H said it was the same circuit. Does not sound the same at all, very touchy pots, foot button 'pops', tone is difficult to dial-in and never exactly what I want. It still does what an envelope filter should do, but I would prefer my original unit. Hard to locate stores that have them for demo purposes, especially to A/B them.