I have this setup as well and while you may not absolutely need an amp its better if you have one. Depends on the source whether its definitely needed or not. For example, the optical output of my U-Verse cable box is not nearly loud enough to rock the music channels if you don't have an amp. Personally, I would not recommend the Q701s without an amp. Different strokes for different folks.
Oops, I guess worded it wrong, the Q701 does need an amp, just not when you have the X-fi HD to power it.
Edited by spongeworthy - 3/14/13 at 11:14am