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.