As a few users already suggested, you can connect from either the speakers / amplifiers using high level input to a subwoofer, no issues. I have a pair of REL connecting this way.
Subwoofer Basics Help
I am thinking about adding a subwoofer to my system but don't fully understand how I would connect it to my Rega Osiris amp which doesn't have a second signal output other than record out. Will a sub work through the record out path? If not that only leaves me the option of using the speaker out terminals. Is there an issue running the sub off the same output as my speakers Naim NBLs? Will my NBLs be compromised? If I have to run the setup this way am I looking for a passive sub? What is the price point that gets you into a decent sub?