I have a streamer with dac. Not sure if a mini dsp is possible since it goes between digital source and a dac.
It usually goes between the pre-amp and the amp, or the source and the pre-amp.
(Best on a higher line level side, rather than a cranked down pre-amp).
So on a tape source side would be common.
MiniDSP is but one example.
Because stereo is easier to deal with and ht has many more spkrs. Not sure many use minidsp.
Car audio is also an area where is it hard to put in draperies over the windows or other treatments. Most of that is 2 channel, and the use of DSP is standard.
Stereo is not easier to deal with, other than 2 is a lower number than say 5. And 2.0 is a lower number than 5.2. People usually do better listening to a single speaker, 2 hides sins, and 5 hides sins even better.