connecting subwoofer via speaker terminals

@lloydc hasn’t said if his sub is equipped with speaker level inputs, and if it is I endorse using them, as @gdaddy1 and @fastfreight have suggested. The cases where this is not advisable are with monoblock amps (unless using a sub for each) or with some Class D amps that don’t have a common ground, like NAD. 

OP has also not said if his amp has a processor loop or pre-outs and amp inputs, so incorporating an active crossover like @elliottbnewcomjr suggests may not be an option. Plus, in my past experience these can place an unwanted veil on the upper range, as few of these gadgets meet high end preamp standards. 

Thanks for all the responses! I do monitor them.  I will look for subs with speaker-level inputs.

I am actually thinking about giving this a try.  My Yamaha in 
"Pure Direct" does not send signal to the subwoofer, so if it to be, it has to be off the speaker terminal.   Other modes do include the sub.

I have a Sunfire Signature that allows for this and in fact can take both channels to make a mono signal.  But I would prefer to keep them separate and if I could find another Sunfire Signature at reasonable cost (need not be working), I would scoop it up.

These are repairable for ~$300 so long as the woofer and passive are not damaged.