Need some Amp help - a little new to properly powering speakers


Hello. 

 I have some polk LSIM707s that I thorough enjoy.

However, at the moment I'm powering them using a Yamaha aventage 3070 receiver which at 150 watts at 8ohms sounds pretty darn good. 

However, since these are rated at 300 watts at 8 Ohms, I assume I will need some more power. I notice at lower volumes a lot of the imaging and clarity disappears. 

I am looking at buying a 300 watt emotiva Amplifier, or a 500 watt emotiva amplifier.

I'm assuming it would be better to purchase the 500 watt per channel emotiva so the speakers won't suck it dry or stress it. 

Am i wrong in this assumption? 
moskaudio
I would only suggest 30 watts if the OP only listened to polite music and only listened at polite volumes. I couldn't find measurements on this specific speaker, but another in the range dips to 4 ohm with a -47 degree phase angle. I would expect the larger speaker to be more challenging to drive. That, combined with the lack of sensitivity, would make me consider a bit more power than 30 watts. My 32 watt amp is just barely competent to power my 92dB sensitive Focals and I'm certain I drive that into class AB on a regular basis. 
Hi Al,
 Thanks for the additional specifications,  I was relying on information from the Polk website. "If"  the tube amplifier has good quality transformers and power supply (PS) I feel the same as atmasphere.  This type of tube amplifier using its 4 ohm tap has a very reasonable chance of success. Atmasphere's recommendation for about 20 db of NFB (or thereabouts) makes sense as this would lower output impedance and conversely increase the DF (Damping factor). 

Meeting this criteria and the need for quality transformers/PS is the challenge for the stated budget for such a tube amplifier. 
Charles 
30 watts was a minimal recommendation to provide moderate/reasonable listening levels. All 30 watt amplifiers aren’t created equally. There is a hierarchy of quality that definitely exists. For example I would be confident that the VAC Renaissance 30/30 amplifier (excellent output transformers and PS) could drive this speaker from its 4 ohm tap and produce beautiful music while doing so. Problem is you won’t find a used one anywhere near 1K dollars. A used Cayin, Line Magnetic or Quicksilver in the 50 watt range push pull class A/B would work pretty well I believe. Interesting thread.
Charles
Moskaudio, am I to understand that you are now considering replacing both the speakers and the amp and going to a 2 channel system? If so I would suggest that you search out the desired speaker first, pair them to your Rotel stereo amp, which seems as no surprise to me to be superior to the Yamaha AVR, while your explore the market for an upgrade in amplification. Finding speakers that are at least moderately easy to drive maximizes ones budget towards amplification. 
@moskaudio What one needs to understand from the get-go, is that the number of watts is only one part, one quite small part, of the story.  For starters, take a look at the current output rating (not all manufacturers publish) and the damping factor.  Generally speaking for both, the higher the better.  Then you get into the jungle of class A vs. AB vs. D, and solid state vs. tube.  So not even two amps with identical measurements on all those parameters will sound the same hooked up to the same set of speakers.