Nice write-up on the GS-1. I had a GS-1 w/ DACT before upgrading to electrostats, and it's still my favorite solid state headphone amp. I didn't have a chance to try too many headphones on it but it paired really well with the SR-225i. The sound was tight, fast, and clean. No veiled, muddy sound that I often hear from sub-$500 and portable headphone amps. Bass was nice and impactful, drums had a good depth to them, and it could handle very fast passages in metal well.
I'm surprised the GS-1 doesn't get more attention. It's a great performer, Headamp products are always well-built and reportedly well-serviced, and does what it's designed to do without any fuss or complication. If I was building a work rig, it'd probably have a GS-1 or Super 7.