So my final guess, which is much more realistic I think, is this:
We saw in the Line 6 promotional video all the fine capture equipment they used to measure and capture the Metallurgy cabs. I suppose that whilst they were taking their detailed measurements they recorded speaker impedance curves at the same time.
So probably the HX Cabs now all have proper SICs embedded within them and that, coupled with improved power amp modelling in the amp models means it would sound more accurate and indeed "better" because the amp model power section would change its behaviour based on selected cab + SIC.
This assumes that all amp models had fixed baked-in SICs interaction behaviour before. Now that could be a wrong assumption, but if that's true, it would also mean that with the existing firmware, if you were to choose a cab that was less suited to a baked-in amp model SIC, then it wouldn't sound so good. Perhaps that is what's behind the years-old argument between Helix users where some say that the stock cabs sound good, whilst others don't like them - because outcomes are higly dependent on what combination of cab + power section you choose; sometimes you get lucky!
And yes, before anybody points it out - I'm kinda obsessed with SICs the moment!
