Wet/Dry amps into headphones, no problem!
The short: Disappointing out of the box but there's plenty of fine tuning in the software and the end result is a very natural speaker representation.
The long: I bought the Iconoclast solely to use between amp and headphones for quiet playing. I've used a digital modeller with various cab sims and also an IR headphone-out from a tube amp but they've never sounded right even with settings and EQ's tweaked to death, this made headphone playing disappointing and fatiguing. When I first plugged the Iconoclast in it sounded pretty much the same as my previous experiences, however after connecting to a PC and spending some time adjusting the software settings, while A/B-ing between speakers and headphones, the end result was a very close match to the speakers. That sonic match isn't just a snapshot though, it's across the spectrum of volumes, distortions, effects, EQ's, pickups etc. the Iconoclast somehow manages to react like a speakers would across the board. But that's not all, the stereo effect which is off by default makes the sound more spacious and more like speakers in a room rather than beamed into your head through headphones.
I presume my experience out of the box is down to there being so many use cases for the Iconoclast that it can't be set by default for everyone. I'm running 2 tube amps wet/dry with the headphone-out of one amp into one Iconoclast input and the second amp into the second Iconoclast input via the line-out from an attenuator, this seems to make the stereo effect even more effective and is a very good representation of two speakers in a room run wet/dry, really good! I run the headphone-out from the Iconoclast into a cheap mixer which I can choose to add music or drums to play along to, then the mixer runs into a cheap headphone amp. All that works great but before I got the mixer and headphone amp I ran straight from the Iconoclast and it has way more output than my 120ohm Sennheiser HD560s headphones need, volume is not a problem with these!