I would recommend the next headphone in the line the AD900 as they go for very cheap now. They're an improvement in all aspects of the AD700 especially the bass and they're also smaller and much lighter. You can also replace the pads of your AD700 or if you get the AD900 with the W5000 leather pads as it improves the bass trememdously. The only problems I have with the AD900 is that the drivers aren't angled so they touch your ear (It takes some use to forget it) and they're styling is very minimal, quite utilitarian actually
For the soundcard and DAC/Amp I would recommend using a Behringer UCA-202 or 222 interface control instead of a soundcard or DAC as theres a RCA analog output on it and it connects to the computer via usb. For the amp I would recommend getting one from audio-technica as they were designed to use with these headphones, any of them would be an excellent compliment to the headphones.
And any specific amp recommendations?
and would the m50 be better?
The M50 has recessed mids. The AD900 does everything better than the M50, especially sound stage, except the M50 has more bass. The M50 is just a step away from being a basshead lite can.