TOPIC:

I've been using a miniDSP 2x4 and REW with stereo subs for a few years. I have been pleased with this setup but I'm on a bit of an upgrade kick and I wondered if replacing the basic model with a miniDSP 2x4 HD would improve the sound. What do you think?

I should clarify that the miniDSP is in the subwoofer circuit only. I run my main speakers full range. 

I doubt if you would hear much, if any, difference.

I would try running your main speakers through the miniDSP as well.
That gives you the option of crossover filtering and/or EQ for the mains.
And, it also removes the 1 millisecond relative delay the miniDSP 2x4 is introducing in your subwoofer channels.  (That would probably not be significant in most applications though.)

The 2x4HD has a significant feature addition to the standard 2x4.  Streaming audio capability to the USB port. 

Dave.

I have a high performing exaSound DAC and very revealing speakers. Running them through the basic ADC and DAC in the miniDSP would definitely not be an improvement. I sure wish my DAC had a digital output though. Bypassing the ADC conversion would be a clear win for the 2x4 HD. 

I have tried filtering the monitors through passive FMod, electronic Marchand XM-44 and line-level passive Marchand XM46, and in every case the loss in fidelity was audible and not worth the higher achieveable volume. I rarely listen beyond 85dB and my speakers are not stressed at that level.

Have you tried basic vs. HD 2x4, or are you just looking at the specs? You may be correct, I wouldn't expect an improvement going from 48kHz to 96kHz would matter when I'm crossing over at 63Hz, but there are other differences. Not all ADC's and DAC's have equally great bass, I suspect mainly because of power supply. There may be a disadvantage to the miniDSP 2x4 because it has a voltage divider in the power circuit (5-24V allowed). 

A voltage divider?  I don't think you understand how these gadgets work. 

Regardless, if you are solid in your beliefs, then you shouldn't be passing your bass through these devices either. 

Dave.

Maybe it's not a voltage divider, but there is clearly an extra electronic circuit in the miniDSP 2X4. It only needs 5V, yet it can run with anything up to 24V. Something is needed to drop the voltage, and it's not simple a resistor. Someone on DIYaudio measured distortion on the miniDSP and said it was lowest when fed with 5V. 

My sealed Bamberg S5-MTM speakers are specifically designed to work with a sub. The miniDSP is a compromise compared to the other gear in my system, but it more than makes up for any sonic penalty by allowing me to implement a Linkwitz Transform and room corrections. My Crown amp with miniDSP absolutely murders the plate amps I used previously.