Yes, yes indeed! This very topic (the two Mullard types) has been the topic of conversation going back to the LDII++ amps. DoA and I feel the same on this matter (we've talked about it) and many others like you have found out the same thing. There is definitely something superior to the full shield M8161's that is hard to put your finger on. It does exist though, and it's audible for most users. Perhaps it has something to do with the tougher build quality internally. We may never know exactly what it is caused by, but I would still recommend buying the full shield M8161 over the "OEMs" or half shields.
Also, I'm glad your source improvements have worked out great!
That is not to say the small shields are terrible. I sometimes prefer their more airy presentation with my Senns and Goldrings. And I still prefer them overall to my Tung Sol/Nationals which get no play from me.