I just built an 18w Plexi and it's working well. I had erroneously believed that the HI vs. LO inputs were controlled by the 1M resistor that is added to the HI jack but not on the LO jack. However, now that I'm looking at adding a VRM, I see that I need to move that resistor to the other end of the input cable (input of V1) along with adding a DC blocking cap. Once that 1M resistor is moved, it is clear that it impacts both HI and LO inputs.
I exchanged a quick email with Stephen, and he confirmed that HI/LO is controlled by the 68K input resistors. After some Google searches, I'm struggling to find an accurate description of how that HI/LO switching works. Is it the wiring of the Cliff jacks? Does the LO input actually engage both the HI and LO input paths in parallel, while HI is isolated to just the HI input path?