I did not find NE-6/NE-7m (both are the same, I heard the NE-7m) to be bass "heavy" just that it's bass lingers on a bit producing a thicker mass. It won't reveal subtle details in bass. But I find the quantity to be hardly worth the "heavy" tag. It has very good sound stage (relatively speaking among the three) and a signature that is non-fatiguing. Technically speaking, it does not reveal too much detail, but I did like it when I had it on loan.
PL50 does not have bass, good mids, good treble, good sound stage and imaging... but, I generally find Soundmagic's signature of PL30 & PL50 to be good, but not special. I can see many loving it, but I don't think I can live with it, perhaps because I've heard a few better IEMs.
M2 has forward mids, good bass, decent treble, not so great sound stage, but has that non-fatiguing as well as engaging signature.
Purely my opinion (and I did not compare them side by side, as PL50 and NE-6 were loaners and my M2 is a constant loaner from my side)
>= is Minor Difference
> is Major Difference
Overall ranking : M2 > PL50 > NE-6
Sound Stage : NE-6 > PL-50 > M2
Bass Quantity: M2 > NE-6 > PL50
Bass Quality : M2 > PL50 > NE-6
Mids : M2 >= PL50 >= NE-6
Treble: PL50 > M2 >= NE-6
Engagement: M2 > NE-6 > PL50
Details: PL-50 > M2 > NE-6
Comfort : PL-50 >= NE-6 = M2
Build Quality: M2 > PL50 = NE-6
You can live with any of them. Just decide! Now, if I am wrong in any of my above stated opinions, others will fix it for you.