You are on the right track getting a sub. 35 watts is probably not enough to take care of the entire bottom end, but if you roll them off at say 80hz to a subwoofer or two will amaze you! If you go with REL or similar, that use your amps speaker feed, you shouldn’t need anything else. They can control where the speakers roll off and the subs roll in.
Let us know what you decide to do.