I had looked into this when I used to run a Pass XA30.8 and was contemplating adding a sub.
From the Pass Labs Owner’s Manual regarding subwoofer connection: “Some powered subwoofers require an audio signal ground reference, and their makers may suggest that a black speaker terminal is where this voltage reference might be found. On complementary designs, which would be typical of all power amps produced by Pass Laboratories™, this is an incorrect and unsafe assumption. On all Pass Laboratories™ power amplifiers a reference of 0 volts will be found only at pin 1 of the XLR input or at the shell of the RCA input. Reference connections for these devices require special consideration when used with our product. If you have such a device, and wish to use it with our product, please contact our Foresthill, California factory for specific instructions.... you have been warned.”
Personally, I'd consider using the line-level connection from the preamp output as this eliminates all issues of negative speaker terminal connections and potential conflicts.
From the Pass Labs Owner’s Manual regarding subwoofer connection: “Some powered subwoofers require an audio signal ground reference, and their makers may suggest that a black speaker terminal is where this voltage reference might be found. On complementary designs, which would be typical of all power amps produced by Pass Laboratories™, this is an incorrect and unsafe assumption. On all Pass Laboratories™ power amplifiers a reference of 0 volts will be found only at pin 1 of the XLR input or at the shell of the RCA input. Reference connections for these devices require special consideration when used with our product. If you have such a device, and wish to use it with our product, please contact our Foresthill, California factory for specific instructions.... you have been warned.”
Personally, I'd consider using the line-level connection from the preamp output as this eliminates all issues of negative speaker terminal connections and potential conflicts.