Thanks for the update, John. Glad to see you cherishing the sweet music these phones can offer (particularly when driven as well as from your set up).
"Stax SR-009? Who cares?"? I reckon you would if you had the chance to try one and compare but, yes, for the extra dollars involved would the additional increments of detail add so much more to your current enjoyment? Probably not (but that doesn't mean it mightn't make a difference for others of course - YMMV).
Great to read that your 'taking one for the team' in acquiring the Sigma Pros has been of benefit. Now you've got reasonable reason to sell your Sigma 404s if the Pros serve you better (unless, like me, you like to have a back-up - in this case should those difficult-to-replace 1 um Lambda Signature drivers ever fail).
Happy listening, John! A chance to re-explore your music collection in total again LOL!