^Ive heard the RX700 which Sumchamp owns and I must say it's pretty impressive.It has a really good thumping bass,airy and smooth midrange ,decent highs ..the detailing,clarity,soundstage etc is also pretty good. The RX 700 is a totally worth the price.
It sounds better compared to my Philips SHP 9000 when connected directly to soundcard .(yeah I am that lucky guy who got the 4k cans for 2.5k

).The SHP 9000 are very difficult to drive and I need to connect it to headphone out on my MX-5021 dock which has amplification for headphones.The only reason I've still kept the SHP 9000 is cause they are super comfortable and light and when connected to a amp they outshine most other headphones in their range.
No idea about the RX-900 but from what I've read they are heavier than the 700 and less comfortable.I myself found the 700 itself to be a slightly uncomfortable for long hours as they completely surround the ear and the pads are not made of material which will let your ears breathe.
Otherwise the RX700 are totally worth the moolah , especially if you are adventurous enough to perform some small & easy mods on the pads.
I would say save the money and get the RX-700.You can invest the rest of it in a amp maybe.Adding a E5 to the RX700 will only increase the volume and bass.I've tried it .Get the E5 only if you feel the volume is low.
Good choice with the XONAR DX .Its a really good sound card for music and you will have some space for future speaker upgrades as well. The MX5021 sounds awesome with the DX.
