hey there,
here are 2 options you can get from my point of view that you can use with asus xonar
75W RMS Logitech
http://www.newegg.com/Product/Product.aspx?Item=N82E16836121044&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
50W RMS Creative
http://www.newegg.com/Product/Product.aspx?Item=N82E16836116049&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-Speakers-_-Creative%20Labs-_-36116049&cm_sp=&AID=10440897&PID=3938566&SID=
I'd definitely go for Logitech for quality budget audio systems. If you want a better quality stuff... I'm sorry but $200 wont be enough... For this budget these are the most suitable options.