op
in hi fi hi end audio there are usually complicated answers to various equipment questions
for 2 channel music with dynamic box speakers ... rel is always the right answer (not the only answer, but never the wrong answer, often the best answer) - that is what they do, they do it properly and their quality (after brief lull in the the late 2000’s when they moved production to china) is excellent
a pair will generally do wonders, but in a small room, if you are patient with setup and follow their instructions meticulously, one may be enough -- remember, use the high level input with speakon connector
no affiliation to rel (just have had roughly a dozen of their subs over the years, a pair of storms now in main system when needed)
properly implemented subs in many, if not most 2 channel systems is revelatory