MiniDSP 2x4HD used with a high efficiency system

Anyone had success using a MiniDSP 2x4HD in a very high efficiency speakers (104dB) horn speakers without "hiss"?

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?
belyin…  Thanks for the input.  Yes, if I use only as a low pass on the base cabinets the Oris are dead silent.  I was trying to give them a hi-pass filter to keep the sub 100Hz off the drivers to extend their life.  Good idea on the PS, I'll give it a try.

Out of curiosity, what drivers are you using with your 150's?  I'm trying the new Voxactiv AC-2.6's and really like them.  Which base cabiner are you using?
tinear . . . I am using AER drivers which replaced Lowther DX-3's. The Voxativ look amazing but out of my price range. On-the-other-hand, the AER's now need refoaming, and I am not sure I have any option other than sending them off to Germany to AER for what seems to me to be a very high price. The bass cabinets are JBL 4648's from their professional cinema series–two 15 woofers in a ported cabinet. I never thought about blocking the low frequencies in terms of preserving the drivers; I am certainly no expert but I it seems to me that the natural roll-off provided by the horn loading eliminates any deep bass response from the drivers, and they are designed to play full range in any event. That said, there should be some benefit in limiting the low frequencies in terms of the horn's amplifier performance which could be accomplished with a simple passive line-level filter in front of the amp. 
belyin... Once I get farther into the tuning I'll give you some feedback on the Voxativ AC-2.6 drivers.  I do like very much the AER's but have had a hard time keeping them alive for any reasonable length of time.  I looked at a number of driver options.  The Voxativ line spans from ~ $1.5k/pr to of course $25k/pr much less than the AER DB-2 or 3.  The AC-2.6 is the middle 8" and lists for a sensible $3.7k/ matched pr.  For the base I am using Bert's BD-15 Reference Compact design at present but am considering change.  I really like the transient response of the La Scala "W" horn.
tinear . . .  I bought my AER's lightly used quite a while ago. I think they correspond to DB-1 in their current line-up. Do you know anyone in the States who can refoam them? I saw a post somewhere recently that related someone's effort to do it themselves, and it sounded daunting. I have never heard true horn-loaded bass in a home setting--as always one picks one's compromises! 
I heard of someone servicing AER in the DC area a while back.  Let me see if I can find the info.  The tolerances of these drivers is very tight.