I have the Fiio E7 I use for my iPod and I'm now extremely tempted to get the E9 based on the price and what others are saying about it. I also have a Little Dot I+ which is the only thing that is keeping me from ordering the E9 immediately, as I am very happy with the sound. And it is slightly less expensive than the E9, with the opportunity to try different tubes and opamps, which I enjoy...since that allows you to change the sound slightly for minimal amounts of cash. But if solid state is your thing, Fiio looks like they've got a real winner with the E9.
The LD I+ has more than enough power to make my K702's sound amazing. And with the Amperex tubes I threw in for 30 bucks, it helped the bass quite a bit in both quantity and quality. Soundstage is also W I D E which I personally love
So you're looking at a ~160 investment. What I will say about the LD I+ is at a recent meet, I got a few comments on how well it held up against some of the really expensive amps there, which I thought was cool.
However keep in mind that the little dot will add a slight amount of warmth depending on the tubes you use while the solid state E9 is very neutral. The slight warmth of tubes might help ease some of the sharp highs that the DT990 sometimes produces.
Either way, with the price of these two amps and the value you get, you really can't go wrong with either. Both produce gobs of power so whichever way you go, I think you'll be happy. It really just comes down to preference.
