Preamp out to line in?

You can run the pre-out for the front channels of the Marantz into a line level input on your Parasound.  If the Parasound doesn't have a theatre pass through, you'll need to find a position on the volume knob of the Parasound that works with the level of the other channels on your home theatre receiver.  When you run home theatre, you'll have to set the volume of the Parasound at that volume position and use the Marantz to adjust the home theatre volume.

I basically do that myself except my LTA preamp has a HT mode that locks the volume level at a certain level.  I use the channel level adjustments on my surround receiver to get all of the levels to match.

The Halo P6 has dedicated inputs for a HT processor/receiver. Look on the back for "Home Theater Bypass Input"

The bypass mode disables the Halo’s volume control and keeps everything in place for the HT receiver/processor.  This is necessary to ensure the channel balance remains correct. 

The P7, a very nice preamp, had 7.1 channels of input for HT and would let you keep the preamp volume control.

@erik_squires  Nice catch.  Ignore what I said.  lol. 

Hey @erik_squires thanks a heap, I feel like I just got a Christmas present. I'd been pondering for days how to do this and as soon as I ask, there's the right answer! It's even got a connection for the sub!

You are most welcome, glad I could help.  Of course, I could only give you such an easy answer because you picked the right preamp.