MiniDSP 2x4HD used with a high efficiency system

Sorry my friend.

I'm using them, and they sound good but with a typical sub.

What exactly are you trying to do?

I'm using as a xover (hi pass to Oris150 horns and low pass to the woofers) crossing over at 200Hz and a bit of room EQ.  

My chain is PSAudio BHK Signature Pre --> MiniDSP --> Coiencident Frankenstein II 300B --> Oris 150 Horn.  Then additionally MiniDSP --> Crown K2 --> Woofers

I can hear a hiss when the MiniDSP is in the signal chain from the highly  efficient horns. 

Can you measure the hiss? 
 
Have you tried bypassing the BHK (digital ?

I only get the noise floor when the MiniDSP in in the chain.  I'm going to go back through the DSP setup and reduce gain to see if there is any effect.

I use the MiniDSP with my Oris 150 horns, but I just use it as a low pass filter to the bass cabinets set at160hz with a second order filter—this matches up well with the natural roll off of the horns while providing a clean signal to the horns.  I suggest you give this try if you haven’t already; I believe this is how the Oris designer intended it to be used. 

MiniDSP nows sells a higher spec version, but I don’t know if is any quieter. Have you tried a better power supply?