The Polk’s are really nice speakers for the money! They aren’t that difficult to drive, but the impedance does drop down to 4 ohms in a couple places:
http://www.stereophile.com/content/polk-rtiii-a1-loudspeaker-measurements#jvJVoa14obi9cxh0.97
For this reason, I would avoid the HD-AMP1. Sure, it’s a small footprint and has the features, but it has a much smaller power supply than your Rotel RA-12. The sound will probably be somewhat weaker than what you have already.
As far as the other two choices, I suppose it really depends on if you want everything in one unit. Personally, I would lean towards the PM8005 because it has a slightly larger power supply and would supply more grunt for the speaker (i.e. bass and midbass would be a bit more powerful/impactful). It would also allow you to flip on your sub at times, like you said you do. Yes, you'll need to buy an external DAC, but this option will ultimately give you everything you're looking for.
The Marantz stuff will probably be a lot more musical sounding than the Rotel. As far as DACs, take a look at the Gustard X-20 as well.