I just spent half an hour trying to work out why I was hearing comb filtering on my snare drum. I'd recently switched to a system of aux-tracks for mixing and was worried that Sonar wasn't doing delay compensation properly, which could have accounted for hearing comb filtering if the snare was arriving at the master bus via 2 routes, one slightly offset from the other. My suspicions were heightened when I muted the reverb bus and the problem stopped.
 
But I decided to verify this by soloing that bus, and that's when I found out that the DRV-2080 was introducing comb filtering on its own. I tried several different presets but it was still there, at least on most of them. Then I swapped out the DRV-2080 for ValhallaRoom to see if there was something intrinsic to having an effect in that part of the chain, but no, ValhallaRoom sounded perfect.
 
So, maybe it's just part of the intended algorithm, but it sounded very wrong to my ears, and I just wanted to have this on the forum so that others might be aware of it.

Was the plug correctly set to 100 percent wet?

Yep. I checked that first. But I realised afterwards that it would be a really weird thing for a plugin to not correctly align its 2 signal paths anyway.

I figured some pre-delay might be a factor. But you're right, that would be weird. So is this though! Hope you figure it out.