The Panasonic class D amplifiers require a very stable eight Ohm load, no dip lower than six Ohms, to sound their best. As the impedance of the load rises or falls the sound quality of the amplifier will correspondingly shift. Higher impedance will typically give a dull sound with compressed dynamics and rolled off top end. Lowering the impedance gives a glassy hardness to the sound and compresses almost every aspect of sound quaity. Phase angle of the speaker should be kept to a minimum. Personally, I would suggest a single driver, full range speaker but they do not always suit the taste of someone listening to "loud" music or complex orchestration. There are a few designs available on line which employ a full range driver supplemented by a tweeter crossed at a higher than average frequency (5-10kHz) which give good results with the Panasonic amplifier. Buy a speaker with reasonably high sensitivity as the class D amplifiers tend to produce aluminum spit balls when driven into clipping.

I have an XR55. I too was intrigued by comments about the virtues of its sound. At first, I found the sound harsh and brittle, but after a few days it lost the harshness and brittleness for the most part. At very loud volumes it becomes very unpleasant.

I listen mostly to the XM radio service through a Polk device. For a slightly livelier sound I have the "re-master" setting turned on to the extended range setting. I do some listening to CD and keep the "re-master" effect engaged.

The speakers I have are a pair of Wharfedale Pacific Evolution 20(tweeter, 6.7" woofer) column style. I am sure that there are better speakers, but they suit me fine.

The only real question I have about the XR55 is that I wonder what 150-200 watts would sound like. However, Panasonic is keeping the XR5x at 100 watts.