What do I like about the Polk Audio Lsi speakers? It would be easier to state what I don't like about the Lsi9: finding 24-26" stands that won't tip easily with the tall, 33lb speaker, and occasional bass boominess.
The boominess is only present with a few recordings, but at a certain bass frequency (I haven't isolated) and those recordigns with a thick bass signal in that frequency range. Otherwise, frequency output is well-balanced.
Any shortcoming in the bass is by far surpassed by smoothness, detail, and life-like tonal qualities. With a good recording, the speaker is never harsh or causing fatique when played at high volumes. Its been noted in reviews of expensive bookshelf speakers that poor recording quality is magnified by the speaker's detailed rendering, but I find that the Lsi9 is more foregiving of the harshest qualities, such as sibilance, distorted snares and cymbals. Crackle, pops, and hiss from old timey recordings will probably be more noticeable with any high end bookshelf.
With high quality recordings, the Lsi9 captures the subtlies in the instruments, such as the breath traveling into the wind instruments, the tautness of the drum membrane, or the silky, metallic ring of drum cymbals. Instruments sound more life-like than the less expensive speakers that I've owned and heard.
Another brand that I *highly* recommend is PSB. I own a pair of CS1001 outdoor speakers $550 pair, which resemble PSB's Image indoor model. The PSB is another very well-mannered speaker that renders plenty of life-like detail. It can't play as loud as the Lsi9, or reach the level of detail in the uppermost frequency range, but bears healthy resemblence otherwise. The detail and dynamic punch is excellent. The Imagine B is easier on the amp than the Lsi. Though your Denon's should power both with no problem, they would probably run a little hotter providing current to the Lsi because of its lower nominal impedence.
Crutchfield has a 60 day money back guarantee with free shipping and I recommend that you pick up a pair of PSB Images and compare side by side with the Arx. Other than jacking up the credit card cost initially, you've got nothing to lose. Its the best way to make a decision when buying mail order.