Mad Dogs all the way. A very nice sounding headphone all the way around - and also very comfortable. I haven't heard the K550s, but I have heard the Q701, K701 & K240. I liked the Mad Dogs better. YMMV.
I wouldn't get too hung up on the amp. Try it without the amp, then take your time, save some money and watch the buy/sell forum for a deal on an amp. Or, just try something cheap like a Fiio E6 or a CMoy just to see if it makes a difference. If it doesn't seem to make a difference for you, then you can go and enjoy your headphones. Don't let the "you have to have an amp" bug control your decision for the headphones. Listen for yourself. The reality is that a decent, but inexpensive amp that is properly driving good headphones will do MUCH more than a great amp driving mediocre headphones. People badmouth inexpensive amps because they bought a really cr@p amp like a Boostaroo, or they tried to drive hungry & completely inappropriate headphones with a small portable amp, or they put really cheap headphones on a decent amp and then complained it didn't make a difference. Try to drive a 300 Ohm 'phone off your iphone, then put a $30 Fiio amp in line, and you *will* hear a significant difference - even if it just in volume!