Too bad the SA31SE isn't a fully balanced amp... even though it has a 4 pin XLR connector for balanced headphone hookup, it still uses a single-ended topology internally (from input to output). I figure if you're spending over $300US, why not try to get an amp that has both balanced and single-ended outputs? Of course the Lyr 2 is just SE as well. And if you step up to the Mjolnir, it's a purely balanced amp - and you can't use SE adapters with it (read the product FAQ for the warnings).
I'd look at the NFB6 - it has both SE and balanced headphone outputs and is a fully balanced design. And it costs a little less than the SA31SE as well. The only advantage I can see on the SA31SE is the volume control with it's much finer resolution. I'm going to continue saving my money and eventually pick up a Bryston BHA-1 - that's the best solid state headphone amp I've heard yet. One of these days I'll make it to a big Head-Fi meet or something like RMAF/CanJam and get to try/hear the Headamp GS-X Mk 2. The GS-X is quite a ways out of the price range I'd normally look at, but from everything I've read, it's the "cat's meow" of solid state amps, able to drive any headphone with impressive neutrality and power.
Sigh, so many options, so little money... :)