Those look amazing! Great job in (somehow) jamming both DD in that small shell. I AM glad you opted to not go with the MX500 shell (even though they are a bit easier to tune in that shell). Are those metal shells or plastic shells? Can you liken them to anything you own, or have you created something completely unique?
Looks like that might take a while. My order has yet to be even processed (as of this morning). Just as a reference point, I ordered them on the 6th. Though, I am not complaining; it isn't like I don't have anything to listen to while waiting for them....
Even though I don't have the M5 Pro yet, I am going to go out on a limb and say that there probably is a difference. If you look at the M4 shell, there IS a port hole in it, but there is just very little room even in the back of the shell where the port leads to. This will stifle the bass to an extent. Where as the M5 pro (and the M5 regular) shells have a "maze" type of configuration much like the ST10S and ST20 (and pro) did. Having said that, I would also guess that the M5 (either shell) should have a bit less bass impact than did the ST10S (and maybe the ST20 also) because of the modular design not leaving any room directly behind the driver.
For me, the ST10S B&G is one of their bassiest models (that I have heard). The reason is the same for your lack of hearing the same bass between the M2s Pro and the M4. Though they are both modular in design, the M2s Pro have lots of room directly behind the shell which creates more room to "breath" so to speak.
Having said all of that, even with the M2s Pro, the differing bass quantities is more subtle than it is "night and day" different. Don't forget that with the M5, they now have added 3 more drivers to play around with (150ohm LCP, and 50ohm LCP, and 32ohm BE) that they didn't have before. So, the list is added to, though I am not able to find all the drivers on their site that they previously had listed for the M4. I don't know if they have discontinued some of them, or they are just not listed (both their official site and Aliexpress sites are a bit of a mess at the moment). To my recollection, they have these available for use with the new M5 pro and M5 (though I may be wrong about the driver coating, they have so many differing ones):
- St20 100ohm Bio
- St20 40ohm
- SO 100ohm Bio
- ST10S 150ohm TI
- 40ohm LCP (stock M4 driver)
- 32ohm TI (I think this one was discontinued not long after the M4 came out)
- M5 Golden 150ohm LCP
- M5 50ohm LCP
- M5 32ohm BE