We have rented 5 Audio-Technica 600 series wireless headset mics. We have 4 of them working perfectly, but the 5th one is causing us problems. It's jumping recievers, causing feedback, there is tons of interferance on the line and so forth. Any basic problem that you can encouter with them, it probably has done that. Now, thes mics work on the the 400 mhz frequencies and for anyone who knows waht they are, they are all A band frequencies. Each reciever/transmitter can be set to one of ten channels and we've tried them all, excluding the ones that the other 4 mics are on, of course. Does anyone have any ideas as to what can be going on, or did we just get a really crappy mic mixed in with the good ones? On another note, I have 6 other wireless mics (I can't remember the brand, but I think we got them from Rockford...) that we have hooked up to compressors and sometimes, the mics don't work, but it works when I remove the compressor from the channel. How do I make it work with the compressor? and if anyone can give me the basic of how compressors work and how to use them properly, let me know.