I've had the opportunity the past couple of days to commit some hours to this, and I have things to report.
For starters, I'll confirm what I suspected going in: comparing the CLR to the PC+ (PowerCab Plus) in its "Flat" (FRFR) mode, playing presets in Helix using impulse responses from Ownhammer and 3Sigma, the CLR has the edge. I'm not sure how to describe or quantify what quality it is that gives it that edge, but it is just more.... pristine? Present? Haunting mids? So, really, no surprises there. The CLR remains my mental benchmark for FRFR performance. It's just a really, really good speaker.
That said, the PC+ is no slouch. It sounds great in "Flat" mode and if I weren't A/B'ing it with the CLR I wouldn't really have anything negative to say. Where the PC+ really shined though was when I turned off the impulse responses in the Helix and used the PC+ "Speaker Model Emulations". Different from a traditional IR, they model the speaker and driver specifically rather than a cabinet, mic(s), etc. The result is advertised to be a sound less like listening to a mic'd up cab, and more like having the cab in the room. I didn't really expect to be "wow'ed", but I really was. The first chord was kind of a jaw-on-the-floor moment. Very impressive. The sound is obviously different from what I otherwise have dialed in, so it is not consistent with what I hear in headphones or what I feed the DAW, but it is different in a really appealing way as far as monitoring in-the-room goes. It made me not care that it was different, which I guess is as good a thing as I could possibly say about that.
So, given that feature, and given the fact that the next Helix firmware update promises the ability to control the PC+ settings directly from Helix via the "Line6 Link" (AES/EBU XLR), I'm 100% in for the PowerCab Plus. Have I mentioned yet that two of them sound completely outrageous in stereo?
Once I can figure out how to approach packing and shipping the back-breaking beast that is the Atomic CLR cabinet, I'll be putting it up for sale. No idea what to ask yet, nothing to compare it to as there aren't any for sale anywhere on earth. I'll post here on the forum when I put it up. In the meantime if anyone is particularly interested feel free to message me.