Whilst I agree that it is possible to get subs to work well especially in an active system using digital crossovers I reckon a better solution would be Tannoy Eatons:
https://www.audiovisualonline.co.uk/...571da19d3650e0
They won't mind that the room is small nor will they mind being jammed up against the wall or even into the corners. And bass will be more than sufficient. They'll make the Spendors plus a sub sound like toys. Okay they are bit spendy but the original versions do come up second hand for a lot less.