^OK, now that's a more carefully phrased question more likely to elicit the answer you're looking for.
The last major Denon QC issue I'm aware of was the bad batch of NIC boards in a few models a few years back such as the 4310. The threads were full of reports. My 4310 suffered this failure. Denon repaired it under warranty. IIRC posters reported that Denon was replacing the boards under warranty for months even after it had expired.
Now as far "just dudes on the internet" (LOL) here can tell, all other Denon models of recent vintage are of good reliability.
If you're considering a 4520, definitely call jdsmoothie to get the best price.
"JD" – AVScience Sales, 585-645-1006 or 877-823-4452 ext 103
Monday – Friday 9am – 9pm EST (leave msg; also weekends when on forum)
Authorized dealer for Denon, Marantz, Yamaha, Klipsch, Def Tech, etc.