@joss Thanks for that. Need to read in detail soon. Did not put much detail in first post because wasn’t sure whether you are as knowledgeable about HiFi as you are on PCs. So… PC not directly involved in this; only used to put digital feed into amp when I want to listen to Amazon stuff.
I know choosing and connecting a (sub)woofer is a thing to be done carefully. Especially when using relatively powerful amps. Do not want to blow up speakers and/or amps nor cause fires. The amp I have is integrated into the speakers: Quad 9AS models. Nominal power rating of 100w RMS. Max output says 200w in the manual. Have not been able to find out how this relates to ohm rating (understand this is part of the calculation). Believe the speaker part of the Quad are 10 ohms but not sure and having trouble checking.
Understand that woofers can be powered or “passive”, ie run off amp signal without need for external power. I think I need only a passive one. Moreover, do not want to blow windows out. What I am trying to ascertain is what ohm rating I should be looking for in the speaker and, indeed, whether I should be checking its wattage rating, if it has one. So, will go read the stuff you’ve sent and get back to you. Thanks again…