Output to Amp set to Amp Output = Distortion

[moff]

Sorry if this question has been posted before - I did do a search but didn't come up with anything useful...

I've been doing some experimenting and discovered something odd (or maybe it's that I don't understand). I've taken my 11R to a gig and a couple of rehearsals now, and have set it up taking a signal from the "Output to Amp" jack into the FX return of a guitar amp (a Fender Deluxe VM) for monitoring purposes (the FOH gets the main XLR outputs). I discovered early that in this type of setup, the "Output to Amp" source cannot be "Rig Out No Cabinet", but must be "Amp Output" instead.

But I've found a problem there, too. Everything sounds relatively normal if I set the "Output to Amp" source to "Rig Output", but if I set it to "Amp Output", it's distorted - clean amp settings sound like there's an overdrive pedal on them. This is using an amp rig that's setup as "clean" (no overdrive or amp distortion). As an experiment, I was able to duplicate this by feeding my IEM system from the "Output to Amp" jack (this was for proof of concept only). If I fed the in-ears with the "Rig Output" signal it sounded fine, but if I use "Amp Output", it's really distorted.

So, the question is why? It's not overloading the IEMs - the signal coming into the IEMs is well below clipping. It doesn't matter how far I cut back on the "Output to Amp" level in the individual patches\rigs, the output is still distorted when using the "Amp Output" setting. It's my understanding that the "Amp Output" setting should give me a signal that comes after the amp modelling and FX, but before the speaker and mic simulation. If so, its seems to me that a rig that's setup without any distortion or overdrive should still be clean at this stage, just in need of a power amp and speaker. Am I just misunderstanding the Amp Output" setting?