I just (in the past week) sold my DQ-LP1. I at one time used it as rodman suggests, for just the 3rd-order low-pass filter, installing a capacitor on the input jacks of my power amp for high-pass filtering, THE way to do it if a 1st-order filter (6dB/octave) is steep enough for your needs. You CAN use the DQ-LP1 to do the same, but you need to match the replaceable-cap value to the input impedance of the power amp anyway, so you may as well do it inside the amp, and save yourself the cost (and potential sonic penalty) of an extra pair of interconnects.
The B4 is MUCH more versatile than the DQ-LP1, providing 1st/2nd/3rd/4th-order filters---both high-pass and low-pass, in 25Hz increments from 25Hz to 3200Hz., and level controls for either (but not both) filter, for bi-amp balancing. As George said, fully discrete, no opamps or ic’s. Cute little bugger, but single-ended only. If you need balanced, you must go up to the Pass x/o, which is a lot more dough.