Hi Tony,
I'm glad that you found my comparison helpful.
Assuming that you're playing a standard 4-string, you'll not notice much difference across the fundamentals(41 - 98 Hz). I play finger style with flats, that are fundamental heavy, and both phones sound good for that. With a plectrum you'll hear the string microphonics more clearly with the M70s. I don't play slap, but the percussive attack will probably be crisper and the mid-bass harmonics(200 - 600 Hz) with round wound strings will be more even and noticeable. There is no harshness or wacky resonances with either of the phones. I think that the M70s really shine when there is a mix of frequencies across the tonal spectrum, no segment dominates or masks any other part.
One of the things that really floored me was the difference in the bass tone on one of my favorite songs. There is a live recording of Cream's "Deserted Cities of the Heart" that has Jack Bruce playing a short scale Gibson EB-3, a growly bass. Jack liked mid-bass growl and often tried to accentuate it. On this song I never heard that growl, even with the M50x(maybe very faintly). I was hearing the string fundamentals that were dominating everything. It sounded good, but when I listened with the M70x I could clearly hear the growl.
The Guitar Center where I bought my phones let me demo the M70x on specific songs on my laptop's music library, so I could compare with the M50x. I have certain songs with particular bass lines that I know well and so it was a good test to do. If you are looking for some phones that would be used almost exclusively for monitoring your bass, I don't know that the M70x would be a good value relative to your M40x. For monitoring a mix of vocals and instruments I would say the M70x would be a great choice. One reason I like the M70x so much is that with just a little EQing I have a one size fits all headphone, good for general listening or analytical work.
I hope this helps. - Jim