I popped by to see Alto again today and had the chance to hear the SST in the same system that I'd heard the Hypex NC500 in before. I did not hear the swap but I was very impressed with the SST in comparison. In that system the SST improved the exact elements I might have criticised that system with before. So, it was smoother, but with no lack of detail at all, in fact probably the opposite, easier to pick out the qualities of each instrument in the soundstage. I'd also say there was an edge to the system at volume in some tracks with the Hypex, this edge was not there now, so no reaching for the volume control to knock it down a bit.
The SST was impressive indeed, not that the Hypex wasn't at all impressive (and is probably half the price remember), but purely on the basis of auditioning of the two, the SST clearly wins on many levels, it's just had that 'goldilocks' thing where it was very musically involving too and I was enjoying it from the word go.
I didn't hear the ABH2. Anyway, thought I'd chime in, as more opinions to read are always good.