Alright, this thing is growing on me. Still won't call it a massive difference by any means... it's still smaller than the jumps from Invicta to Invicta Mirus to Mirus Pro... but absolutely compelling nonetheless. When things are already at such a high level of performance, it becomes more difficult to eke out incremental improvements. But, at least to some appreciable degree, Resonessence Labs has done it once again.
For the cost of $750 to upgrade from a Mirus Pro, I'd say it's definitely worth the trouble. For the $1850 it costs to purchase new compared to the "regular" Mirus Pro, I'm on the fence. On the one hand, that's a lot of cash. The Mirus Pro is already so good that it might be worth just stopping there. In the other, folks spend that much (and more) on things like absurdly thick AC cables, interconnects with magnets in them, and weird room tuning devices that don't really do much. In that sense, I'd rather see someone run Cablyedyne cables for a modest cost, and funnel the savings into the Sig Pro instead. It's going to be a more tangible improvement as far as I'm concerned. Then again, people buying gear at this level might not care about the savings, so there's that.