So I should probably start looking to upgrade the receiver before speakers? Any recommendations?
Well let's consider a few things
1)The yamaha supports 7.1 BUT - doesn't have HDMI. How important is that to you?
2) The back panel of it says it's rated 370 watts, which is somewhat anemic for a 7 channel receiver.
3) THe Polk R50s are rated 90db sensitivity / 8 ohm nominal impedance. With 110wpc amp in 2 channel, they
should be purty darn loud before they even start stressing the amp. They can take up to 150watts RMS.
So with all of the above said, here's my guess: The R60s may have a
nominal impedance at 8 ohms, but they probably behave more like a 4 ohm speaker in terms of low impedance dips. The extra current of low impedance dips is probably what's causing your yamaha to shut down at high volumes, unless these volumes are really damn loud. Either way, it's amp related.
You have to ask yourself what you want - to be louder or to be
better-sounding.
You can get louder two ways:
a) Add a 2 channel amp like the Emotiva UPA-5 (which is actually a 5 channel amp but it's on sale right now so
go get it!) and hook it up to the front pre-outs on the yamaha.
2) Get more sensitive speakers. Whether these speakers will be better depends on the speaker, not the sensitivity, though. For example a klipsch might get louder than your polks but you have to ask if you like the general sonic character of it (and if that's accurate). On that note, a lot of the better speakers in the price range you're looking at have comparable sensitivities to the polks. IIRC Paradigm Monitor 11s are rather sensitive though...they might be a decent one to look into.
A new receiver won't hurt either, especially since your current receiver doesn't support HDMI audio but if that's not a priority I'd definitely look into a new amp first. I'm not sure however how much voltage yamaha pre-outs can do unclipped. Does anyone know if yamahas make good prepros?
Also, if you're listening to your music in surround IE pro logic II or multi-ch 5.1, you're making your receiver work a lot harder than it would in plain old 2 channel stereo mode.