Unlike a traditional A/V receiver these days or a traditionally powered subwoofer, you have a bunch of setup hoops you have to make sure are all playing nicely together to get that QSC working.
1. Wiring. Subwoofer output from Sony, to XPA input. Use the left phoenix connection block. Use the left terminal. Center conductor to + and the surrounding conductor to the 'ground'. Plug phoenix block back into the amp. From the output of the amp, + and - should be connected normally to the subwoofer. Only connect one subwoofer at this time.
2. Switches everywhere! - On the amp, make sure that the attenuation dial is set to '0'. Completely clockwise. Anything else turns the volume down. This is your master volume control for the subs, so during testing you can turn them down some, but once setup, you want to leave it at a volume that is comfortable with the other speakers. Make sure the 'HPF' switch is set to 'OFF'. Do not connect anything to the volume/standby phoenix block, this isn't needed and is there for external control only.
https://www.extron.com/download/files/userman/68-2354-50_C.pdf
and
https://www.extron.com/download/files/userman/68-2354-01_B.pdf
3. More switches - QSC - On these speakers, make sure that the dial on the back of the speakers is pointed to 50. That means it will take 50 watts of power from the amplifier tapped at 70v. If it is currently pointed at 8ohm then it is bypassing the internal transformer and won't work with the amplifier. So, rotate the knob until it is pointed at the proper power tap in white (70v). There is also a knob/switch on the back to turn the low pass crossover on or off. I would turn it on for protection.
https://www.qsc.com/resource-files/productresources/spk/ad/ad-s28tw/q_spk_ads_28tw_usermanual.pdf
4. Test #1 - The Sony is a complex beast, and it's better to avoid using it during this testing. Just connect an audio source, like a CD player, computer, or phone directly to the cables coming off the amplifier. You don't need left/right, just one channel (pick one) and ensure you get audio playback. This will ensure that things are wired up properly.
5. Well, this should be interesting... Sony: The receiver likely has a bunch of settings in it already, and those settings may not be what you need to use. If it has been set to 'NO' for 'SUBWOOFER', then the subwoofer output won't ever produce any audio for you. So, starting on page 18 of the manual, you want to follow the setup procedures as they have listed. Make sure the subwoofer settings are correct in the receiver (page 21), make sure front speakers are set to 'SMALL' (page 19). You can leave the crossover frequency set to 120hz, but can adjust it later (page 22).
https://www.sony.com/electronics/support/res/manuals/W001/W0010540M.pdf
Page 36 of the manual indicates that there is a SWOOFER light on the front of the display that may light up when you feed it a two channel source, and it's decoding audio for the subwoofer output on the back of the receiver.
6. Trouble? - If audio works from your phone/cd/cable box but not from the Sony? Plug your XPA into one of the other analog outputs on the back of the Sony. Try 'left out' or 'right out'. This should let audio pass to the subwoofer properly. Make sure the amp is turned up enough to pass audio as well.
7. Option: If none of that works, just hook the subwoofers up to the Sony on the 'B' output. Make sure the dials on the back are set to 8ohm. Your main speakers are 8 ohms and the subwoofers are 8 ohms. It should be fine with the Sony.