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

Moskaudio
Atmasphere mentioned tube amplifiers in his reply to you. You Polk is rated at 88 db sensitivity and an 8 ohm load. It would be very informative to listen to your speakers driven by a good quality push pull tube amplifier of suitable power. You may find they have more to offer  than you're currently experiencing with the solid state receivers, 
Charles 

@kosst,

The OP stated that it "sounded much clearer at lower levels." He did not say it played "louder."

The fact of the matter is that a fair comparison of amplifiers requires matched levels.

High efficiency speakers manage greater low-level dynamics for a given volume.  His original post stated that he noticed a lot of "imaging" loss at lower levels. Maybe he was referring to imaging in the true sense of the word, but I'm betting that dynamics is the more appropriate adjective. I've never had an issue with low-level imaging, but all mid-efficiency speakers I've owned struggled to produce decent dynamics under 70db, regardless of the paired amp.

 I know...I know...you'll tell me I've never owned any decent speakers and that I apparently haven't heard the Focal Arias...

Helomech, I'm going to say you've never heard LS50's. Those are even less sensitive than those Polk's and nobody has ever accused that speaker of being weak on detail and imaging. You've got opinions about the importance of sensitivity just like a bun h of horn fans do and won't be swayed by reality. Lower efficiency, reactive speakers need more power and more damping factor. Not what you get with receivers. 

You do not need more efficient speakers for more detail. The most detailed speakers may well be Quad electrostats, and they are horribly ineffcient. The Harbeth range is also very detailed, at both high and lowe levels, and they are not particularly efficient either. All that such speakers need is power, and the more the better.

@Helomech, dynamics are the more appropriate adjective. There was some mid range and high range loss, but anything below -35DB on the Yamaha receiver (150 watt @8ohm) (which again, is a $1,999msrp receiver), had a LOT of dynamic loss.  

Then when paired with the Rotel Receiver (100watt @8ohm), the dynamics were more apparent at lower volumes. As the Receiver measures its volume in DB, the Rotel does not (0-100). Around 30, the Rotel receiver brought out more dynamics, and mid ranges that sounded lost with the Yamaha, at what was a much lower volume. 

I'm pretty new to this, but overall after doing some DB sensitivity research and other things, it's pretty safe to say that Polk LSIM707s definitely aren't the most responsive towers on the market. 

However, @Charles1dad, I think I'm gonna give a tube amp with them a shot. Seeing as how I'm new, the experimentation would be fun =). 

@Charles1dad, what tube amp would you recommend in or under the $1000 range? I don't mind buying used audio equipment on Ebay. I do however live in Oklahoma and needless to say....there isn't a whole lot of after market trading here.