I thought that I'd post some impressions to this thread now that the gs-x mini is in the wild. I have serial #3. This is a fantastic headphone amp. Here is a few pros and cons, off the cuff:
Likes:
1. Outstanding parts and build quality. Fit and finish are exceptional. Aesthetics are often a matter of personal preference, but I have to admit i find the look of the Mini to be dead sexy.
2. Functionally this amp is very versatile. The built in pre-amp feature works well. Conversion of SE sources to balanced output is quite handy. And then it powers just about any headphone on the market (dynamic and planar) equally well. I love the volume tracking on this amp. It's very tight and precise and gets louder gradually...never in a harsh way or overbearing.
3. Tone of the amp hits the sweet spot for me. Instruments (particularly the ones that I play personally) sound convincingly real. Vocals are authentic and have a layered presentation to them. I can hear the low raspiness of Patricia Barbara when she belts out a tune in The Cole Porter Mix. Fine detail is a hallmark of this amp. I supposed it's function of the quality of the PSU among other factors.
Dislikes:
1. Honestly, I have a hard time poking any holes in the gs-x mini. I guess if you must have a tube amp, you have to look elsewhere. There are some very fine imperfections in the top plate in the powder coating. But that's about it.