Hi. I'm from Brazil too. And I also have a Xonar Xense combined with a pair of HS80M. What a coincidence! :)
What you need to plug these monitors in your Xense is this. Just plug the P2 (i.e. 3.5mm) on the front speaker output (lime color) of your Xense, and each P10 (i.e. 6.4mm) on each HS80M. It's very simple.
And about the hiss coming from the tweeters, it's true, and it doesn't have anything to do with the cables or the sound card. In fact, you can hear it even without any cable plugged, and doesn't matter the volume you set. More about this here and here and here and here and finally here:
Hey fellow HS80M owner here! I noticed the same problem when I got mine. Yamaha USA's position is that the slight hissing results from the active speaker design. They wouldn't send me a replacement pair as this is a documented issue.
It seems that there's nothing we can do about it. :(
BTW, que tópico é esse que você leu? :)
Edited by Delerue - 6/25/12 at 5:44pm