I got a great deal on a sennheiser lav set, but it came with an EK300 IEM receiver. I assume the biggest difference is that I will lose some frequency response, and I would be using it at very very low gain on EK300 dial.

It seems to work fine and well plugged into a 5D MII and it seems to be noise-free as well.

Should I spring for an EK100 receiver or EK500 receiver instead?

Thanks

All the impedance and gain structure will be wrong as the IEM receiver is designed for use with very low impedance headsets etc, it will also be very hot signal wise and just turning it down will increase the noise that is recorded.

It is useable but if I were you I would get a variable Pad to reduce the signal level down by at least 30-40 db's and if it sounds OK then you are good to go.

I have personally used an IEM system as a wireless distribution solution but as the audio was purely for guide or PA use I didn't really notice much frequency loss.

An EK100 receiver would be a better solution as it has the correct levels and interface connectors, you can always still use the IEM at the same time as a wireless monitoring solution.