I've spent about a week now with my new DAC paired with the Crack and S.E.X.
Here's the update I've just added to the first two review posts...
DAC Upgrade Update - 27th October 2013
A recent upgrade to my DAC has brought some very interesting changes.
The new DAC is a Matrix X-Sabre based on the very detailed ES9018 Sabre DAC chip. The extra separation and clarity offered by this DAC (compared to the previous Audio-gd NFB-5.2) has brought the Crack (with Speedball) very close to the S.E.X. in terms of overall sound performance. It's so close now that I could actually be completely happy with either amp when driving the Beyerdynamic T1s. For lower impedance cans, the S.E.X. is still the only choice due to the Crack's high output impedance, but with high impedance Beyerdynamic or Sennheiser cans I would probably choose the Crack simply because it is cheaper and still excellent.
I think the key to this change is the signature of the DACs. The NFB-5.2 was slightly warmer than the X-Sabre. The X-Sabre is still musical, but a little closer to neutral than the NFB-5.2. The result is a cleaner sound from the Crack - still warm and rich, but not smoothed over at all. The S.E.X. can sound a little dry at times in comparison which makes the Crack a bit more seductive - it soothes the ears with detailed, mellow tunes while the S.E.X. presents oodles of details and clarity, but isn't always soothing.