Edited by bmeat - 8/19/14 at 9:34am
Each has there own postives, the K240 are good music headphones, but needs a basic amplifier to sound their best,
Have not used the HA-RX900 in years (they are in pieces), so hard to remember their sound.
The Hi2050s are almost as good sounding as the HSR-1000, being the Hi2050s are semi-open, might be the better choice for FPS gaming.
Brainwavz HM5 have a good clarity, but not much bass.
The MDR V6 is the best I've heard at it's price range. There are minimal improvements as you go up in price (though only some improve). I'd recommend the V6, though it does sound very different than the HD598. Buy the valour pads and they are EXTREMELY comfortable. The most comfortable closed can I can remember (more comfortable than the 1r). They're here: http://www.amazon.com/Beyerdynamic-DT250-Replacement-Earpads-Velour/dp/B003Y59WNE/ref=sr_1_4?ie=UTF8&qid=1382639406&sr=8-4&keywords=dt250+velour+pads
I agree with you. These are an excellent headphones for the price and also comfy for a long listening sessions. Changing pads to the velour makes these more comfortable (ears don't sweat so much), but they ruin the bass. It becomes boomy and uncontrollable.
If you have not tried dac/amp yet, it is worth trying dac/amp along with HD 598. E10 is a budget dac/amp. If you need portable solution, you can go for E17 or E 07 K.
Sound quality of my K240 Mkii is enhanced by E17. If you do not believe me, look this thread also. http://www.head-fi.org/t/687344/new-here
Have you looked the like that I posted earlier.
Most of on board sound cards are craps. That being said, K240 or whatever phone you are gonna buy might not make night and day difference in sound quality. My suggestion is, why dont buy budget dac/amp such as E10 and see if you like the sound quality of HD 598? I also wonder, why you do not want to go for dac/amp?
E10 is 75.99 in amazon and E07 K is 89 $.