From the manual for the X350.5:
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.
A way to implement this, assuming that the connections between the DAC and the amp are via their XLR connectors, would be to solder the ground wire from each sub to the ground sleeve connection of an RCA plug, with nothing connected to the center pin of the RCA plug, and to insert that RCA plug into one of the RCA jacks on the amp. It would probably make no difference sonically whether you use separate RCA plugs for that purpose for the two subs, or just a single plug with the ground wires from both subs connected to its ground sleeve.
The (ungrounded) signal input wires from the subs would of course be connected to the positive (probably red) output terminals of the amp. Since you are intending to have two subs, you would probably want to connect both the left and right signal wires from the left channel sub to the left channel positive output terminal of the amp, and both the left and right signal wires from the right channel sub to the right channel positive output terminal of the amp.
Alternatively, an acceptable approach MIGHT be to connect the low level inputs of the subs to the RCA outputs of the DAC, while connecting the Pass amp to its XLR outputs. Before doing that, however, Weiss should be asked to confirm that those two sets of outputs are designed to be used simultaneously. It is conceivable that the RCA outputs are driven by the same output stages that drive one of the two signal polarities that are applied to the XLR connectors, in which case this approach would not be a good one.
A third approach, if the sub provides XLR inputs (such as some of the JL Audio subs do), would be to use XLR y-adapters, such as these
, to route the DAC's XLR outputs to both the Pass amp and the subs. The low output impedance of the DAC probably makes that a more reasonable approach than it would be with a lot of other equipment, although it is still conceivable that this approach could compromise, at least slightly, the quality of the sound that is heard through the Maggies.
The latter two approaches, of course, would sacrifice the sonic advantage that was mentioned by Pierre of having the power amp in the signal path to the subs.