asus xonar stx is worth buying only if you have a Rs.20000+ speaker system.you seems to be mistaken about sound quality.the most basic factor in sound quality is speaker.if you have cheap speaker it doesn't matter even if you buy a Rs.10000 audio card but on the other hand a Rs.20000 speaker will still sound great/better than any cheap speaker even without a sound card.
That is totally wrong. The input you provide to the speakers is what matters. Specially considering when you have a decent speaker.
but then again i would like to add that the difference in the sound quality may be very noticeable / may not be . So you need to try out the set-up for yourself.
and getting a high-end sound card has its advantages. IF you listen to music a lot, then after some time the sound quality on any audio system decreases and then you can think of spending a good amount of money on a good pair of speakers.
And to be very frank, spending 9k on a sound card is not easy for everyone, so just be really sure before you spend that amount of money.
@OP i feel it is stupid when you are using only 2.1 channel to get a 7.1 channel sound card? Also the sp2500 already features a DSP. !!!
You should seriously look for a high end 2.1 channel audio card. Try auzentech prelude, Lynx L22