In 1992 I built my first 4-way active system.
To minimize the hiss introduced by a NADY 4-way 24db per octave crossover I did the following steps based on the concept of minimizing gain applied to noise inputs.
(1) Use a CD player with variable outputs.
(2) burn a cd with max volume (0db) sign sweeps and put that in the CD
(3) use an oscilloscope on the cd player output to find the highest gain setting without clipping at any point on the 0dB sweep
(4) connected the crossover and checked each output for clipping. Increase the input gain to max without clipping. (seems it was ok even with gain maxed)
(5) checked each output of the crossover and found the max no-clipping output gains
(6) connect the amplifiers, move the scope to the amp outputs and reduce amp input gains until clipping is absent.
(7) connect speakers except for tweeter and balance the outputs for SPL by reducing amplifier input gain.
(8) reduce the output of the cd player by 20dB or so. Set tweeter amp to minimum gain.
(9) Attach the tweeter. Increase the tweeter amp gain until it's output is balanced with the rest of the drivers.
Steps 8 and 9 are used to avoid burning the tweeter out.
Steps 1 through 7 create a high initial signal level that allows your amplifier gain to be set at the lowest point possible and still be able to reach max power.
With the amp gains at this lowest practical setting, you minimize the amplification of noise from the crossover.
It seems this approach might help reduce noise of your DSP units.
Also there is some possible performance improvement if you replace the single amp and high level passive crossover of the horn driver with a dual amp and passive low level crossover made up of capacitors and resistors. maybe like this:
Currently I use a source with XLR outputs. These "pro" units are usually able to output 14dB higher than "consumer" signal sources. That means a big improvement in signal to noise from the very beginning of the signal chain. There is a jumper in the miniDSP that needs moving to use this option.