The 701s need a decent beefy phone amp to drive them properly. If i swap my 701s for my Grados through my PIMETA head amp the level i listen to the 701s at comfortably, is ear shattering through the Grados
i have the same phones running through a stock digi, they don't get super loud but definitely loud enough. If you need them going louder than that, you are seriously in danger of messing up your ears if you are going to be using them everyday. I hear the M50's aren't as accurate, so if you're listening for pleasure, it's cool....but if your listening for accuracy, I'd stick with the k701's.
It really will make a big difference. I noticed mine out of the box were closed, felt restricted, thin, and one dimensional. Even after 10 hours break in they will improve a lot. Give it a go and you'll see what I mean. After 2-300 hrs they will be completely different phones. Just leave them on as long as you can (also give them a rest for an hour every 24 at least) and take notes so you have a kind of history to look back on. They can go through several stages of break in, actually at around 250 hours or so if I remember correctly they start to sound worse again but quickly come back on form, so don't worry if that happens.
You probably won't be able to make important balance/tonal decisions before they're properly broken in. Any hours will help but I'd give them at least 100 as a start. Mine are at 400ish and in daily use even though I'm sure they're still not fully broken in yet - but they're done enough for serious work by now.
I used a playlist consisting of various sine sweeps and pink noise tracks, plus music tracks of different genres. You should probably set the play level at a moderate level first and increase it as the hours go by, but not too loud. The drivers on the K701 are pretty tough but they need to be taken care of until they've got a few hours on them.
Put them on something shaped like a head so the sound resonates inside the ear cups as it would on your head. This will soften up all parts of the phones. Leaving them open on a desk (allowing the sound to travel out from front and back) will probably only break the drivers in, which is ok, but it won't help to settle in the rest of the phones. It's the same principal as new guitars - you need to play them a lot for everything to resonate and the wood to settle in.
I have both K701 and the M50's and feel that they actually compliment each other very well..they're almost at opposite ends of the headphone spectrum in most regards..and as such,between the two of them you can get a pretty balanced picture of what's going on.
I would confidently mix just using these two cans.
Re:the K701's..and as has already been mentioned,there's nothing wrong yours they're just very inefficient headphones so will sound ALOT quieter than the M50's on identical settings..not a huge problem in general but you wouldn't want to use them straight out of your ipod for example.
I have a little-dot headphone amp which I use with mine,if necessary-very reasonable-(considering the crazy prices of some of them)-and works a treat!.
...the M50's,on the other hand,aren't fussy at all.
Re:burn in-I did a very similar thing to mark,various genre's,(with an emphasis on 'big',wide freq,dynamic productions)interspersed with white/pink noise..and gradually increasing in volume every couple of days.
I didn't actually use sweeps because I read somewhere they could potentially damage new phones if played too loud during burn in...but I know many people do use them..