Just wanted to help here. The new Ref series has two single-ended cable in the lineup: SEI-600s and SEI-600II
SEI-600s is a twin-breakout of two coaxials that are made from the core coax within the SEI-600 original. Thus, the sound of this cable will come closer to the sound of the original SEI-600 because everything is the same except for the connector (which is built the same way, just more termination friendly) and the fact that the secondary shield is not there. This is useful in a rack situation where components are relatively close to one another. As this is a stereo-breakout cable, it's not necessarily the best solution for monoblocks. ;-)
SEI-600II combine the twin coaxes in parallel, summing shields to shield and signal-conductprs to signal. Lower resistance, half impedance, half inductance. This has a very different sound from the original and the 600s. This is a very excellent cable for runs out to monoblocks, but many prefer its sound in the short-run through the rack as well.
Hope this helps!