One of the basses that I play is the U-Bass, from Kala. It has a piezo system under the bridge, similar to many acoustic instruments. In the U-Bass fan club on TalkBass, we are often on the search for the perfect preamp that will allow our beloved U-Bass sound good. One of the things that many of us have discovered is that a 10MOhm input is pretty much a requirement. The piezo quack is best tamed at this high of input impedance.
Recently, Radial Engineering introduced the PZ-DI. It appears to be a distillation of a few essential features from its much larger brethren, condensed into a DI box about the size of a 125B pedal enclosure. Of particular interest to my needs and for my testing was the adjustable high-pass filter (HPF) and a fixed low-pass filter (LPF). The HPF cuts the signal at -3db/octave and ranges from 5Hz to 500Hz. The LPF starts at 3kHz. Between these filters, my U-Bass signal was effectively contained, allowing me to adjust for the boom of a room/stage and reduce the quack.
Overall, I found the PZ-DI to be a good product. The overall sound quality while playing direct is fantastic. It's flat frequency response allowed the tone of my U-Bass to come through without having to worry about a mids hump or drop. The construction is solid all around. The product feels like you are holding a brick in your hand, thus it might make for a good self-defense weapon in pinch. The only thing I find missing on the PZ-DI is a level knob, but I'm not sure where it would fit.
The PZ-DI's input impedance is adjustable: 220K, 1M and 10M. Thus, I also tested it with my fretless Gretsch G2202 and was again very happy with the sonic results.
In all, I am very happy with my demo testing of the unit and almost don't want to return it, except that Radial has my credit card number lest I absconded with the product. For bass players that want a simple setup when playing direct, the PZ-DI should be worth considering, especially if you play an acoustic instrument.