DX3 Pro+ & PA3s pair better than MX3s:
For headphones? Probably.
For stereo? Not sure.
As standalone DAC or preamp? MX3s can't even do those.
Yes, but do you even need it? That's for you to decide.
That's more of a philisophical divergence than just a price difference.
DX3 Pro+ is an AIO device that does MANY things great: Standalone DAC, pre-amp and headphone amp. It's up there with SotA systems for just $199, even less used or with discounts. Yet if you have passive speakers, you're still going to need a stereo amp.
MX3s is an AIO device that does ONE thing (presumably) great: Driving passive speakers directly through of digital input. There are other AIO devices fitted to that niche under $200 like the great performing
Loxjie A30 for ($170 new/$140 used) for example.
So it depends if one wants versatility of a 2 component setup or simplicity, compactness and affordability of a single unit.