No, because you are already running it without issue and the specs for the mains is tha same for the center, actually for all channels, so I would think it is the same amplifier and we all know Denon has a rock solid amp section.
The thing is with the 4 ohm load you are now getting even more wattage but with higher distortion ratings, I would guess around 1% at max output. This is where an outboard amp comes into play. If you are listening at high volume levels that actually get the amps to near max output and audible distortion, an outboard amp will supply cleaner power(low distortion) to reach those volume levels.
