Parasound P5 + A23 vs Musical Fidelity M5si

Go with the Musical Fidelity M5Si.
My advise is since you already have a streamer/DAC you should be looking for an analog integrated amp. I recommend the Rega Elicit-R analog integrated amp currently retails for $3k new. There are other options out there. 
Naim makes really good quality streamer & DAC.

I would go with the separates.  So many possibilities for later upgrading.

I would agree with the idea of separates due to many possibilities for upgrades down the road but the separate gears that the OP is considering are mediocre at best especially for his B&W 804 D.
I’m pretty sure the Musical Fidelity M5Si will outperform the Parasound Halo P5/A 23 combo. Musical Fidelity makes very good & refined sounding gears.

However, since the OP already has Naim streamer + Naim DAC, which are very good digital front end source components, I would recommend getting an analog integrated amp instead. He won't need a built-in DAC in an integrated amp or a preamp since he has the Naim streamer + Naim DAC, which will outperform any onboard DAC in any preamps or integrated amps anyway.