I'm glad for you to find the pads to be satisfying. the Yaxi is one brand that's quite popular here and are pushed quite heavily by one local retailer, I was one of the people that tempted by the advantages, and IMO it well deserve the price. It fits not only the M40x, but also most studio Headphones, ie Sony MDR 7506 (V6); Shure SRH; AT's M series (other than M70x). etc. Well, you'd know if you have the packaging in your hand. They also made microfiber ver of it BTW, they say the material is even softer and smoother than the Alcantara that's on Sennheiser HD800, Hype? I don't know as I Haven't tried that one yet, just some info if you ever want to go that route.
About the M70x, I'll just leave my note of the Impression I have when I demoed it, beware that it is quite rough, and YMMV:
"M70x: It sounds Fuller, more technical and better separation than M40x. Overall it has this "magical" feeling on it's mids and treble. Very treble focused but not harsh with fuller mids than M40x and easier to drive, It just sounds right in any music I throw at it…
Direct comparison with MSR7: Unlike M70x, I can hear some harshness to the sound of the MSR, I dont know why some people find it the opposite; IMO M70x is the acoustics king with smooth and clean sound sig, while MSR7 is more for broader genre, especially like pop songs
I quite like it actually, It IS lacking in the bass quantity, but extension, impact, and speed are not bad. With Q1: With Bass boost the bass amount is just enough, not really looking for it anymore… It's a dream combination"
The way I'd describe M70x treble area is more of a gentle hill rather than mountain with peaks and cliffs, They are treble focused for sure, but they are so very smooth and detailed, I dont find them harsh in any way at all. Mind you this might be because my ear just are not sensitive to the sound that people find to be particularly harsh, each people's hearing are different after all.
I do like it though, and am planning to replace my M40x to it in the future.