Not sure about the E9 sounding empty, but at the price range, I personally find it to be quite potent (I'd recommend spending the extra $20 for the E9K aka E09k as it does away with the useless 3.5mm input which was horrible on the E9, and has al full sized RCA input/outputs. I also found it to not have a noisy volume pot, which I had a problem with on the standard E9.) The HE400 doesn't need much. I have used quite a few amps, and sound quality between SS amps doesn't change as much as head-fi in general wants you to believe. Keep an open mind, and be realistic, and you'd probably think the same.
I downgraded from the SA-31 to the Compass 2, and I'm just happy if not happier.
Edited by Mad Lust Envy - 2/15/13 at 2:52am