I don't think you have all your facts right in your first post. For example, the AVR-X3600H specs also claimed the ability to drive speakers with impedance 4-16 Ohms.
Both units can no doubt drive 4 ohm speakers under the right conditions. A quick way to get a rough idea is to use an online calculator such as this one:
https://myhometheater.homestead.com/splcalculator.html
If in doubt, it wouldn't hurt to add a 3 channel amp for the LCR speakers.
The Onkyo's specs does give me the impression that it has a stronger amp section, probably more comparable to that of the X4500H, though the difference in terms of SPL will be very minimal, may be 1 to 1.5 dB at the most. You know you need double the power to gain 3 dB spl right?
Both have the same AK4458 DAC for the main zones and a lower grade one for zone2/3.
Since you are in Canada, you should know that the Marantz SR7013 was on sale for $1,249.99 less than two month ago. It lasted for about a week, then the price went back up. Obviously I cannot link something that is no longer there, but you can see guys spreading the news at the time over at the redflagdeals.com.
https://forums.redflagdeals.com/best-buy-today-only-marantz-sr7013-9-2-channel-4k-ultra-hd-network-av-receiver-1249-99-2301967/
When the sale was on, it was available at both Bestbuy and Amazon.ca, for the same price.
My point is, if you can wait, and you prefer Denon, then you should wait patiently, because is that kind of sale happened once two months ago, it will likely be back. The Denon AVR-X4500H and Marantz SR7013 are very similar, the Marantz adds 7.1 analog inputs and HDAM, that's about it.
