HDMI switch and an external amp would be my choice...
I was in the same boat a while back. Like you, I run a larger Polk tower for left and right mains. My HK receiver did a fair (at best) job of running these towers at low-medium levels. If I wanted to open up and run them loud, I would always have to reduce bass/treble levels, lower individual input levels, and hope that I did not cook off one of the drivers. Low power + high gain = distortion.
In an effort to give my RTi10's some juice, I researched every damn company I could. SQ and value were my targets...emotiva seemed to offer both but further reading revealed issues of build quality and sound quality...leaving value in my court. Mind you, this was just my research and I blame the copious amounts of cold IPA & pissed off Emotiva owners that drove me away from a purchase.
My money went to the purchase of an ADA PTM6150. This a was a craigslist find and the owner bought it new in the late 90's but failed to unbox it for the project he had in mind. I scored quality and value in one 25 minute drive from my front door. I brought it home, terminated pre-outs to amp inputs and powered my system up. Right off, I noticed how much better the sound was at low volume levels and raising the level only became better. My entire RTi system now sings where it used to hum.
Invest your dough in power. IMO, your current Yamaha is a fine processor...open those Polks up and enjoy the sound.