I have a few sennheiser old ew100 g2’s that are in the 600 range. They are regulated as backup equipment. If I needed 1 in an emergency, I could use them in the gap.To be legit, you should buy new mics. However, if you rarely obey the speed limit, that would be some indication that you will tolerate some risk. If you use your tunable mics in the duplex gap, you will probably never get in trouble. I just wouldn't book any gigs for FCC employees.