I wish I could tell you about the A-100, I ordered one late last month, but shipping got delayed and I don't have it yet
If you don't plan on powering speakers and can afford it, I'd suggest getting the Massdrop THX. I've read only good things about it and with that, you won't be in any need of upgrading your amp for a long time, unless you want a different flavour of sound. The atom also seems good for the price. I enjoy my Massdrop Liquid Carbon, but if I had to choose between it and the THX, I'd go THX to future proof myself.