This reply is months late (from when you asked (see above)).
I haven't heard the Sigma HCs. I did own a Sigma Analogue.
Being a Shunyata cheerleader, I am usually doing cartwheels even before I get a
Shunyata component I've ordered. So, if I'm not doing cartwheels after insailling it in the same system, I have to listen to it carefully.
I liked the Sigma Analogue - but I didn't love it. (After a month's worth of listening, of course!)
I DID LOVE the Sigma NR, because I could hear instantly that it brought me closer to what I hear in the concert hall (and it wasn't just the "lower noise,": instruments went deeper into the pppp range (which could be a result of lower noise, I'll admit), but there was more to it than that.
Why not order from Music Direct, which has a 60 day return policy (although these days, I'm not so sure about that: everyone's concerned about the virus) and listen to it and see if you hear a difference? It should be fairly obvious. Or ask Grant or Caelin what the musical improvement is that they hear. Is the pluck of a cello string more "vibrant" or the harmonics on a harp more vivid? In my experience, when the upper midrange/lower treble is improved, the entire frequency range improves. More hall ambience, faster transients (on the technical side), but also more of a sense of "reality" (again, as heard in a concert hall, which is what my training was when I was at Fi and TAS). I don't compare components to components, because...well, I'm listening fora reproduced facsimile of the live musical experience, and there's no substitute for real life. And NO component is perfect.
Not having had the HC, I don't know. Others will have had more experience.