Hello,
I have been using a 4520CI for the past month and it really is a special AVR. While I cannot speak for the 4520's amplifier stage as I am running it in Preamp Only Mode, the preamp stage is astonishingly good.
For the past 5 years I had been using a succession of Onkyo AVR's starting with a TX-SR805 and a few weeks later a TX-SR875 that I used for several years before replacing it with a TX-NR3007 which succumbed to HDMI Boarditis after 11 months that was in turn replaced with a TX-NR3008.
In all instances I was running outboard amplification for all channels consisting of an Aragon 8008bb, Parasound HCA-3500, Parasound HCA-2205AT, and Parasound HCA-1000a. Upon the purchase of the AVR-4520, my Aragon 8008bb became utterly transformed. This amplifier in particular requires a quality preamp stage as it has a low output impedance and I do not think AVR's being used as a preamp was very high on Mondial Designs list of priorities. This issue was mentioned when Secrets of Home Theater & High Fidelity reviewed the Aragon 8008x3.
When the Denon AVR-4311 was introduced, I did give serious consideration to it. However, I was not particularly fond of the 4311's industrial design and when Onkyo swapped out my jinxed 3007 with a 3008 the addition of Audyssey MultEQ XT32/SubEQ HT was enough to placate me.
When Denon introduced the AVR-4520CI I just knew I had to have it. The combination of a quite pleasing industrial design and Audyssey LFC pushed me to sell of my Onkyo TX-NR3008, Prior to the quad Onkyo era I was using a Pioneer VSX-49txi which also offered an excellent preamp stage. The intervening 5 years of Onkyo ownership had made me lose sight of just how powerful the 8008bb is as my Martin Logan Vantages were utterly transformed upon the completion of the Denon Setup Procedure.
While I remain reticent to use really any AVR to drive 5 electrostatic speakers, the writer of the Home Theater Shack 4520 Review actually is now driving his Martin Logan Prodigies with the 4520 in a Biamp Configuration. Regardless, this really is a wonderful AVR and decidedly the best AVR I have owned in the HDMI Era.
Cheers,
JJ