The good news is the actual headphones are fine - you just need to change things a little. I did a second show this weekend and the shape and size of the venue, and very 'quaint' layout meant the audience were not too deep, but quite wide, and behind the audience, a wall that reflected sound right back. I got my bass amp out of the flightcase, connected it, and immediately had to turn the master down by 3 notches from the settings the previous show. We had a lot of spare time after sound check, so I was able to try at length.
First thing is the enhanced bass response of the headphones, compared to the SE215. On m,y Behringer P16, this mean the bottom end needed near on 90 degrees of turn on the knob to restore 'normal' (for me) tone and response. The expanding foam pads, a black/grey colour do not keep the sound out as well as the light grey ones on the SE215. I have an old spare pair of the 215s with a damaged cable, so even though the tube on the new M6's is larger (hence better bass?) I managed to squeeze the M6 into the 215 expanding foam, and it sealed so much better reducing the room sound to the same level I was used to on the 215s. While fiddling, I did try the 'tree' shaped soft seals. They stay in is the best I can manage - audio reduction is hardly noticeable at all - you can have a conversation with them in - you find that difficult on the M6 expandable foam ones, and impossible on the 215 expanding foam.
With the adjustment to the bass fed to them, and with the better seals, the M6's are a good product. They do distort earlier than the 215s, but that's only an issue because the seals are poorer. Once I stopped the leakage, they didn't need to be turned up so loud. This roughness when driven hard is the only real thing to remember. For our band - layout means that in some venues, I have the guitarist's musicman right behind me, and the drums only a little way away - so I've even been known to play with my left 215 in, even when I have a wedge - festivals being always a problem for us with a heavy metal sound man who I don't trust to mix my IEMs on our vocal harmony stuff - I'll use a wedge because I can always move closer or further from it and hear the stage sound. Having my left ear blocked gives me protection, even if the plug just dangles.
I have some proper moulds for a pair of the 215s these are by far the best for sealing and comfort, the Shure expanding foam seal comes 2nd, the M6 foam 3rd and the latex style tree shapes are just ways to keep them in your ear - offering minimal sound reduction from stage noise.
The Shure SE215s have a balanced sound - eq settings when plugged into iTunes are neutral - no need for any real eq changes.
The M6 has enhanced bass response and a little extra something I cannot quite identify in the mid-range that makes them just a tad harsher - but perfectly usable
The M6 ear wires to keep them over your ears are stiffer and less receptive to gentle bending, so if they pop of the top of your ear, trying one handed to pop them back over can pull the thing out of your ear canal. The Shure wires are just a little more bendable.
I'm going to order a replacement cable for the spare SE215, and the cable is close to the price of the entire M6 package!
My conclusion is that if you have the money - the Shures at their higher price are good value for that money, but the M6 price point and small sound differences makes them a serious contender - especially if you need to buy multiple pairs. The only caveat would be that your monitor feeds need to have some kind of EQ available - I would not wish to split an output to a 215 and an M6 user - this would be a compromise too far. I suspect they've been design to have the old fashioned smiley face eq curve, which would make their sound more understandable - whereas the Shures are just 'flatter'?
Am I going to replace the M6 in my bass case when the new cable appears? No! I'm quite happy to continue using these now I know how to just tame them a bit and improve the isolation.
