The old Speco transformer was not rated for DC voltage on the winding, specifically the secondary. I didn't want to put any such voltage on that winding, because an internal short would put the voltage on the secondary which is connected to headphones. So, in order to ground one end of the transformer we would either have to keep the cathode bypass capacitor in the output current loop, or get the bias another way. I chose to do the latter, using the WE split-rail circuit. That in turn required that there be two independent power supplies, since you can't count on old-stock television tubes to come in matched pairs(!).
The double power supply was technically cool, but the bifilar power supply winding was at risk of internal shorts unless used in specific ways with the right orientation of terminals. That made it unsafe to offer the transformer by itself, which was a frustration to us and to a few customers who wanted to experiment with it. As we started using the same transformer in other products (Foreplay, Crack, and a couple more possible new products) the situation got more frustrating.
The new output transformer is rated for 500vDC on the primary, so there is no problem having bias voltage (20v) on the primary and grounding the secondary; therefor the dual power supply is not needed and a simpler power transformer was developed; it's in the SEX and Tode now and is being deployed into other products as we revise them.
The SEX 2.1 has a "pseudo-dual-mono" power supply, where the last section of filtering is split into two parts, one for each channel. SEX 2.0 has real dual-mono power, which gives slightly more separation of channels - though I don't really think the difference is audible in this circuit. The smaller parts count in version 2.1 prevented a larger price increase, which was important to us.
I've simplified above but I hope that helps!
