miniDSP with separates or Arcam SA30 or NAD C658 plus beefy power amp?

miniDSP has a lot of great little "swiss army knife" type products to help fix elements in current systems.  However, it's likely going to degrade some sound quality unless you buy a linear power supply for the miniDSP unit.  The switching wal-wart power supplies just aren't good enough.   That being said, it's much better to buy a really good NAD or Arcam that has the room correction included.  That way, you're not sending the audio through another device that will need to do an analog-to-digital conversion, process, and then a digital-to-analog conversion.

Concerning the P429 amp, it is never a good idea to buy a multi-channel amp just to bridge it.  For one thing, it reduces the ability for the amp to handle lower impedances on speakers.  When bridged, you need to make sure your speaker is 8 ohms and does not drop down low.  Many speakers nowadays will drop down to 3 ohm impedance in certain frequency areas even though the speaker is listed as an "8 ohm speaker".  Second, you are compromising the design of the amp and forcing it to execute in a "work around" mode.    You're much better off looking for a higher power stereo amp instead.

I'm hoping you realize the Arcam is an integrated amp, so you don't necessarily need an external power amp for it.

Another thing to be aware - the HT bypass modes will typically bypass ALL PROCESSING in the preamp/integrated, including any Dirac room correction.