Okay, I can see some areas for confusion here that will require clarification. Which scenario are you asking about:
1. Use the Crown amp for main left/right speakers and also connect it to a subwoofer with built-in amp. If this is the case, then elevick’s comments apply.
1. Use the power in the Crown amp to power/drive a subwoofer speaker. If this is what you want to do, then you need what is called a "passive subwoofer". It is, essentially, just a woofer in a box with speaker wire connections. They are out there, but they are very rare - as most people will want "powered subwoofers".
There is a passive subwoofer on audiogon:
https://www.audiogon.com/listings/lis9g48b-tannoy-definition-sub-15i-15-passive-subwoofer-19501-subwoofers Parts Express will sell passive subwoofer kits if you want to build your own.
https://www.parts-express.com/cat/speaker-subwoofer-kits/286?N=23647+4294967118+4294965883&Ne=10166&Nrs=collection%28%29%2Frecord%5Bendeca%3Amatches%28.%2C%22P_PortalID%22%2C%221%22%29+and+endeca%3Amatches%28.%2C%22P_Searchable%22%2C%221%22%29%5D&PortalID=1 You can search ebay for "passive subwoofer".
For this passive subwoofer idea to work, you will need some sort of active crossover or HT Processor/receiver that will only send bass signals to the Crown / passive subwoofer.