Now that I've had the StudioMonitor 65 for a couple of weeks, I can say that these still continue to impress me with their wide open sound. I was worried at first at how bright they sounded, that sibilance would creep in, but they've instead begun to mellow out a little and are very pleasant sounding.
The low bass that they can produce is nothing short of mind-boggling, it's smooth and fat. Even at loud volumes (~90-93db at listening chair), they sound solid and thick and I notice no breakup in soundstage or vocals. Piano is clean and without any ringing whatsoever (although harmonic/modal ringing is generally attributed to room acoustics more than anything when you're using decent speakers). Money For Nothing (Dire Straits) sounds fantastic but the song points out one reality about bookshelves. I've yet to hear a set that can produce the chest pounding mid-bass like a good set of towers.
Because they're bookshelves with a pair of 5 1/4" drivers, they don't have the slam and the mid-bass punch that a tower with larger drivers would. Enabling a sub definitely helps but I've yet to listen to bookshelves (with or without a sub) that have that gut punching mid-bass at high volumes. The Denon 3312 is no slouch but perhaps mating the speakers with something like an Emotiva XPA-5 will give the speakers more authority in the mid-bass.
The mid-range and highs, however, are some of the smoothest I've heard. I keep thinking that Definitive Technology needs to come out with a set of conventional towers (not bi-pole and no built-in powered sub) that combine the SM65 and a cabinet below housing a pair or three 6 1/2" drivers crossed-over for bass.
In the meantime, I can easily say that these have met my expectations for use as mains for both music and HT. They sound great!
Did you by chance audition the 65s and compare them to the 45's or 55s? Just curious on your thoughts.