DB/spl level can be very important when comparing closely matched gear. even .5db different can make two same sounding pieces sounded different, but my experience with miniDSP is different, I found the ddrc-24 sounded worse then all my other gear at any and every level.OK, that is a lot of uncontrolled variables.
To do a proper comparison you need to keep the amplifier input sensitivity and gain knob position constant. I agree that using the 0.9 V setting on the miniDSP and lower input sensitivity on the amplifier would allow for a more apples to apples comparison. I would also verify that the miniDSP is not attenuating output at the 0.9 V input setting by playing a test tone and measuring the SU-6 output voltage and then routing the SU-6 to the miniDSP and measuring the miniDSP output voltage.
All that being said the 2X4 obviously has limited output voltage and an objectively rather poor ADC/DAC but I would expect that to manifest itself in increased hiss, not lack of dynamics.
In terms of alternative DSPs the 2X4HD is a bit of step up in performance and would allow you to eliminate the SU-6 and the additional DA/AD that comes with it by taking an optical or USB input (unsure of what your source is upstream of the SU-6). Newly announced miniDSP Flex has much better spec'd analog performance but is quite a bit more expensive.
Honestly one of the cheapest ways to get improved analog performance would be to find a used nanoDIGI like this -> https://www.audiosciencereview.com/forum/index.php?threads/fs-minidsp-nanodigi-2x8-b.29647/ and add 2 decent performance $100 DACs.
Michael
I have been mixing live sound for 15 years and have used a lot of gear. Some mixing boards have EQ's and compression that just sounds so good, a little sounds good, a lot sounds good, other mixing boards work but never sound good. And a good amount of this was with touring/traveling groups, so same guitar, same eq adjustments just on different systems.
I do not know of any DSP as cheap as mini-DSP, sorry.