I just finished doing about 40 minutes of listening tests with the Acoustic Research Classic 18s. To my ears they are the definitively better than any full range speaker I've owned in terms of dynamics (entire bandwidth). These speakers slam more than any I've ever owned. The attack of the bass with "Hotel California" live is unlike anything I've heard in my home before, even with a subwoofer. Due to the acoustic suspension design of the AR Classic 18s the low end rolloff is soft keeping it going audibly below it's 40hz low frequency extension rating. Sad to say, but the Yamaha RX-V3800 runs out of juice before these speakers stop taking it. I didn't get it to shut down, but I did hear it clipping audibly with transients and with Trent Reznor's constant electric noise.
There are some cabinet resonance issues that cause some slight coloring at mid bass frequencies. However the resonance issues could be resolved fairly easily. At the same time, previously owned and present owned floor standers I have have more cabinet resonant issues than these, so I'm going in the right direction.
In terms of high frequency handling, the tweeter are solid. No matter the level of output they keep composure. I did some listening with high frequency material from Cranberries - Faithful Departed album with some woodwind instrumentation that usually sets other tweeters into disarray. They aren't harsh, but they do seem pretty analytical, not really leaving anything out.
Detailed layering is something else I noticed when listening to Loreena McKennitt's "Marco Polo". Several instruments overlay but all are significant in this track. Speaker of lesser quality or entry level may still sound alright with this song but the convolution can prove difficult for speakers that are unable to present it. There's an underlying bass tone constantly present that could be drown out by the other instruments, but on the ARs it was always right there where it was supposed to be.
Imaging is narrow. Vertically and horizontally off axis tears up the sound stage. When listening to these I feel I need be as still in my seat as I do listening to Martin Logan ESLs. Lucky for me my couch is quite comfortable and the music is good.
My poor Yammy was uncomfortable to the touch after 40 minutes, but I don't blame the receiver. I imagine a 4 ohm rated speaker with this configuration gets a little power hungry at certain frequencies. In all likelyhood it probably spends a little more time around 2-3 ohms than the Yamaha would like for them to.
Refinishing the cabinets would probably be a good idea in the near future. My current end cost in these wonderful speakers falls at less than $200. I'd consider that worthwhile considering they've taken the top spot for speakers owned. They are cool, different, rare, and best of all, absolutely phenomenal sounding full range speakers.