Way back in 2008, I listened to a podcast from Contrabass Conversations featuring Steve Rodby talking about gear issues faced by double bassists playing with amplification. One of the things he discussed at length was the problems with impedance faced by bassists. The problem (nicely summarized here) is that many piezo pickups and amplifiers have low (or lower) impedance inputs/outputs. For a bassist, low impedance results in a very thin tone that cannot be effectively corrected using an EQ. Further, if you use any effects (e.g., tuner, volume pedal) the input/output on those effects can further reduce/change the impedance. As a result, even if you have a high impedance pickup, the signal can be influenced by virtually anything in the signal path between your instrument and your amplifier.
Shortly after I heard this podcast, I spoke to a friend about this problem. He suggested I check out the Steel Guitar Black Box by Sarno Music Solutions. The unit is a tube impedance matcher that allows you to vary the impedance using a simply control. The result is a solution to the tone problem created by low impedance. Speaking non-technically, which is all I know, the box allows you to raise the impedance of your instrument as it comes into the amplifier. The result is a thick, rich tone that captures all the subtleties of your playing. I’ve been very impressed with this device. I’ve had particular success with it at rockabilly gigs where I typically have to play at higher volumes and therefore face feedback problems. The box allows me to not have to totally gut my tone in order to eliminate feedback. Rather, I can use my EQ to reduce the offending frequencies and then compensate with the black box to get my tone back.
The unit costs $310USD (plus an additional $10 for shipping) but it totally worth it if you are playing with piezo-based pickups.