[Inquartata]

Well, then it seems you're fairly set on getting the M50s. Hopefully, they will suit your needs. If you still have the S4's and aren't getting the bass, make sure you've inserted them properly and have a good seal. The S4's have bass, and more bass than the M50, in fact. If you aren't hearing it, you've got a faulty pair or a bad seal (or are expecting XB-line kind of bass, perhaps? I don't know).
 
The 750 does to a good job with classical for its price range (and being a closed can, but this is to my ears). Out of the cans I have access to, the 750 would be my second choice for classical listening (after the MS400s). As for build quality, they aren't going to fall apart. The extenders aren't fantastic, but unless you treat them harshly or there's an accident, they aren't going to just randomly snap. Comfort could be an issue, because the headband is flat (not sure why Shure went with that design choice, and that's why I don't use them too much anymore). Anyway, the 750 far outperforms the S4. So does the M50, but in the bass area that you keep bringing up, the 750 outperforms (at least to my ears) the M50s for the slammin' kind of guilty-pleasure bass. You'll just have to decide what's the most important thing to you. But if you're already totally convinced the M50s are your solution, why solicit opinions? 
 
Anyway, good luck on your audio journey! 

 

[Surag238]

Nope, not sure at all.  I have yet to try them.  I am merely going off what I've read.  Many people mention the m50s are better for other aspects of music than the 750...thats all
 
I was hoping the e5 I bought would offer sufficient bass for me.  Suppose it does (enough for me)
 
hwo would you compare hte 750 with the m50s for everything else-classical, rock, etc? 
 
And would you say the xb700 are as good or not as good as the m50 for non-bass emphasized music?
 
If I feel like the m50s are not 'louder' on the bass than the s4 i am returning.
 
Also, I had a very good seal- I mead sure it was in properly when I wor the image s4.  Even then i was not satisfied with the bass.  

 

[Shreknow91]

I can't compare the m50's to anything else b/c these are the only pair of decent headphones I've ever had, but this is my opinion on them:
 
I listen to a lot of techno, hip hop, and rock, and IMO they have enough of a punch and rumble to suit my needs, I also just picked up a Fiio E5 and it definetly adds to the experience all around!
 
What ever you decide, I hope you are happy with what you get!