powered sub to an Adcom speaker selector box

@ cunawabi
You can use the subwoofer you have. You will need a pair of y adapters on the preamp. Then you can run another pair of interconnects to the subwoofer. A product like this will do the trick!
https://www.audioadvisor.com/prodinfo.asp?number=AQM22FHRD

Thank you Yogiboy. This seems to be the easiest solution . I appreciate it. 

Plenty of good advice from the replies. Here's another suggestion that may offer a more simple solution.......

Check out the Outlaw ICBM. It's an active crossover that offers adjustable XOVER frequencies. It is placed between the preout and main in. In addition to many outputs for multiple speakers it also has 2 outputs for subs. So, there's no need for y adapters from the preamp.

The ICBM has been out of production for a while but can be found in the used market. It originally sold for $250. May make your hookup more simple.

Another mention.....do your 901's have a lot of age on them? If so, check on the condition of the foam surrounds. I've been down that road.

Yes one pair (Vl ) is perfect and I refoamed all 18 of the V ‘s. 

IF you go line level to sub, as yogi said

after you split your preamp output into two left and two right, using the fitting he posted, or a pair of Y adapters

https://www.amazon.com/LEIGESAUDIO-Female-Adapter-Splitter-Connector/dp/B07P45DT6K/ref=sr_1_2_sspa?d...

you need to convert one l/r to mono for the sub

https://www.amazon.com/FosPower-Subwoofer-Splitter-Connectors-Extension/dp/B01LZVLBY3/ref=pd_rhf_se_...