It's interesting that you'd make this distinction, which implies that these later models have a different DC voltage requirement than the 300/500/600/700 models of yesteryear. That may in fact be technically true, given that the 1st generation models used 6AA cells for a nominal 9Vdc for their battery supply vs the labeled 7.4Vdc lithium battery used on the JTV. (and, probably, the Shuriken and Standard models as well -- I don't own one of them, so I don't know for sure...)
As we know, battery/cell voltages drift downward as they discharge during use, so there's really a range of acceptable voltages that can be applied without damaging the instrument. I'd not be afraid to apply 10volts to my Variax 500 or 600, feeling fairly confident that the circuit would handle the slight overvoltage. That said, I've adjusted my adapter board to output 9 volts, to simulate a battery. When powered by a battery, there's also a Vmin cutoff voltage, below which the guitar onboard processor will not function. I have a pretty good idea what this range is, in a general sense, but won't throw out numbers that I'm only making an educated guess about.
But getting back to your comment about adjusting the modified circuit in a box designed to replace an XPS A-B box, I would guess that the XPS, given that it's designed to work with any of the Variax models, doesn't know or care if it's a 500 or a JTV that is connected to it. So what appears that a safe thing to do would be to adjust the output of the DC buck converter used in these projects to the voltage that is output by an XPS A-B box. (In my opinion, it's probably not really required to take this precaution, but it certainly wouldn't hurt to do it!)
So to take this safe approach, we need to know what voltage an XPS supplies to the guitar. I have two XPS switch boxes, and they're slightly different. The Line 6 logos are different and the operation of the power LED is also different. One of the boxes came with my original early release Variax 500. The other one was supplied with either the 600 or my JTV 59. I think the documentation refers to the XPS supply as optional, so it most likely came with the 600. I've never even plugged that one in until today, to run a voltage check...
What I found is, the output from these two XPS boxes appears to be a well-regulated, nominal 7 volts DC. Specifically, the output of one of them is rock-solid at 6.98 volts, and the other one is 7.01 volts. That's pretty darned close...
So my recommendation to anyone considering using this adapter scheme to supply power to any model of Variax via the TRS cable, would be to set the output to 7 volts. The guitar shouldn't know any better, and will thank you for it.