There is more to an amplifier than power, you need to read up I've hooked up a 25W Rotel into large floorstander speakers. Sounds great. However I've also used amplifiers in 100W range that weren't suitable as not enough clean power.
The 671 should be fine, just be aware it doesn't have pre-outs so you cannot upgrade power amplifiers at a later date. So if the amps aren't sufficient, you cannot upgrade. Also how these amps are, they're not specced to power all drivens driven, ie a good poweramp can do 200W per channel all channels driven. AV Amplifiers cannot do that, nor all channels driven without one or more channels dropping down.
I've had a problem with a Yamaha driving 4Ohm speakers, 70W but you can tell not even half that.
Both of them speakers are easy to drive, and not a hard load so I can't see a problem. However I don't know if Polk & Yamaha are a good match. Yamaha are a quite safe sound, I'm using B&W speakers which are generally a bit bright and the combination works well.