Actually its somewhat due to your DAC as well. The Metrum Onyx appears to have 6V output from the XLR outs, based on a review that I found on it. This is a fairly high output voltage from a source component. To put this in perspective, the Bryston BDA-2 is rated at 4.6V output on the XLR output. To me the Metrum Onyx is a good design to use with a passive or unity gain volume control. The fact that Metrum makes the Jade model which just adds a motorized volume control (also an Alps RK27) to the identical Onyx DAC circuit board bears this out.
The BP-25 is not an exceptionally high gain preamp so its not completely due to the BP-25. IIRC I measured my BP-25 at around +16dB of gain via the XLR outputs which means around +10dB via the RCA's. I've had pre's on my bench that had +20dB or more of gain on the RCA's, so the BP-25 is more of a low/medium gain preamp to me. If you combine the high input voltage from the Metrum with the medium gain of the BP-25 plus a high gain amplifier and high sensitivity speakers then you will end up with poor volume control that gets loud quickly. If you want to do a test, try using the single ended (RCA) outs from the Onyx to the BP-25 and RCA outputs to your amplifier. The BP-25's gain is a lot lower for the RCA input and outputs. I think you'll find the total system gain to be a lot more satisfying. I realize that this is not a "fix" but it'll identify how the system will respond by removing gain or sensitivity from somewhere in the signal chain.
