Suppose this could go under 'Effects' or 'Amps & Cabs' but I am putting it here... I play fretless and when I can't hear myself, it's a long night. (I am almost always playing with ear plugs, and I am almost always ampless, in the PA. The other guys typically use their amps to carry the house, but I'd rather use the PA to do the heavy lifting than me.) Wireless IEM's ( Carvin EM900 driving Westone UM-Pro30's ) solved that, when I can get a decent monitor mix from the sound guy (e.g. - when I am the sound guy...). Not always possible. But I do always have at least one and usually more DI outputs at my disposal, either from a pedal or an amp head. DI level, however, has not been enough to drive the wireless signal to audible levels. I finally got around to trying this little gizmo: https://www.amazon.com/gp/product/B0002E50MC/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1 -- an ART microphone preamp to boost the signal. It was $30 when I bought it 3 years ago, it seems to be $40 now. It works great. I had to use the +20dB and turn the knob a good bit to get a usable signal but that's what it's there for. A lot of the reviews go on at great length about whether it does or does not add 'warmth' or 'drive' to a microphone signal. In this application - who cares ? Its only purpose in life is to make sure I get accurate intonation information from my playing - the audience hears none of it. Recommended.