I've heard all these but the lp2 (but have heard m100) and I'd rate their sound quality as: K550>SRH840>M50>M100. All have good sound isolation.
So get the K550 unless you want emphasized bass and don't mind slightly smaller soundstage, in which case you should get SRH840 which deliver amazing sq for the price. SRH840 perform very well without amplification (sorry, haven't played with K550 with different amps). If you want serious bass, the latter two will give it to you in spades and still sound great.
I apologise for taking a while to respond. The music I tend to listen to is very diverse. I'm now tending away from the K550 for the apparent lack of lows and generally not very good for bass. It is now down to the SRH840 and the M50, I would like a decent amount of bass. I listen to a wide variety of music varying from classical to dubstep. I also read something about the durability of the SRH840s not being very good (can someone either advise me on this).Any final advice for deciding between these two cans?
Edited by hydroflip - 3/6/13 at 2:02pm