I want to use a Xonar DX 7.1 sound card in a multi-room setup. I'm playing back 4 mp3s simultaneously on the 4 stereo ports.

I noticed that outputs 1-2 are significantly louder than the rest.

What could be the cause for this? Is it that the system is maybe using a surround71 profile and adjusts volumes automatically? If so, how can I avoid that?