Do you know what the maximum output level is? That's the important information that's missing.
Here’s the reply I got from the builder:
The maximum output level of the 1/4” jack is actually very high, but the practical maximum will depend on the input signal plus the gain through the unit.
I usually figure 0.15Vrms as the peak output level of a passive bass (this is probably slightly conservative), and I typically measure 19-20dB of gain through the preamp at max volume with a flat EQ. Let’s calculate the output this way:
0.15Vrms is equivalent to -14dBu, plus 19dB gain through the preamp, results in 5dBu at output, which is 1.4Vrms. The EQ is boosting so you could possibly add another 6dB of gain at typical settings (up to 18/20dB boost bass/treble at extreme settings) but I’d say you could easily expect to hit 1.4Vrms if you’re looking for a minimum maximum level (haha).
I think the maximum input level for the preamp (prior to clipping) is probably around 2.3Vrms, which is equivalent to 9.5dBu, plus 19dB gain through the preamp, results in 28.5dBu at output, which is 20.6Vrms. This is likely far higher than you would ever need, but to get this level of output you would need to run at least 20dB gain in front of the preamp.
Does that makes sense? The 1/4” output should have enough gain to drive most power amps quite well, if that’s what you have in mind.
I intended the 1/4” output to run to an amp (unity gain is around 11-12 o’clock on the volume knob) with the XLR output going to a console or soundboard, but have seen studio guys running 1/4” straight into their converters and using the XLR output to feed an amp with an XLR to 1/4” cable. That works great too!