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I'm having terrible problems with audio feedback when using the MiniDSP2x4HD.

I have a Denon AVR. I'm only using the sub1 output from the AVR and have the sub level set to 0dB (is that too high?). I also disabled Audyssey, set the Front Mains to small and for the purpose of testing frequency output and determining delay set the xover in the AVR to 250Hz...and will then lower it to the best setting indicated by REW's measurements.

I'm using SpeakerPower plate amps to drive all the subs and I'm using the unbalanced inputs on the plate amps since the AVR has unbalanced outs and the 2x4HD has unbalanced Ins/outs. SpeakerPower has assured me the output voltage of the 2x4HD is sufficient to fully drive their plate amps using the unbalanced ports. For that matter, the analog output of my AVR is 1.2V and the default analog input of the 2x4HD is 2V and SpeakerPower has said their plate amps only need 320mV RMS reach full output.

For the MiniDSP 2x4HD, I'm using only input1. Out1 goes to my my IB subs at the front of the HT, out2 is unused, out3 goes to my back row NFS and out4 goes to my front row NFS. The MLP is centered in the front row.

For REW, I'm using my UMIK-1 as the input. The 90 calibration file is applied. My laptop is set for 7.1 audio output. REW is configured to test channel 1.1 and use 1.3 for timing. Everything seems to be setup correctly. I only this morning started having this loud feedback problem.

FWIW I had a Balanced 2x4 before without this problem.

I believe my problem is most likely gain matching but I for the life of me cannot figure out how to deal with it. If I want to run a sweep, I cannot find the right balance to eliminate feedback while having the volume out of the subs at a normal level. Either the subs are too quiet, REW waits for the return channel or there's horrible feedback through the subs that fairly quickly gets really loud. I have to rush to the AVR to turn the volume down. This is incredibly frustrating.

All I want to do is optimize the gains throughout the system, set appropriate delays for the IB and second row subs to optimize the combined sub response at the MLP and then apply as little EQ as possible to get a nice house curve. It shouldn't be that hard but evidently it is for me

Somebody please help
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