Realize this thread is a bit old but I'm going down this rabbit hole as well since the 410 sub x seems to
A.) Perform well
B.) Is available and for reasonable prices
Some problems -
A.) Hornady uses shorter length brass to accomdate the flex tips and still run it in lever guns. So if you're running a lever gun (like I am) you'll have to trim your normal brass to hornady's specs to get it to cylce.
B.) Hornady doesn't publish their subsonic load data because they're using a blend of powders to achieve what they want.
C.) Finding common subsonic powders for this cartridge right now (Tin star, unique, trailboss) is quite difficult.
I've read some people having success with Accurate 5744 - and I have several pounds of that lying around, so I'm going to try and cook up something.
Hornady has published accurate loads for the 410 subx - but only down to 1300 FPS. Assuming they're running a 24" barrel - this load might already by subsonic out of a shorter barrel, or close to it:
https://static.hornady.media/site/hornady/files/load-data/45-70-government-trap-door-410gr-sub-x.pdf Accuarte has a published trap door load with 24.3g of 5744 pushing a 405 lead flat nose 1,394fps out of a 24" barrel - so using that as a minimum charge weight seems reasonable - but I don't think you'll have to get anywhere near that low.
I plan on starting with the 30.3 gr hornady load, and working down to subsonic from there.
I'll update if it works out.