Good sub pairing for my old Alesis M1Active MkII's

So, I've been using this pair of M1Active mk2s for a long time and I love them, but they lack a low end in their current environment (which is just a small home studio involving all manner of instruments that I can get my hands on). My question is: what sub would pair well with them and also be an appropriate match in price? I could drop some cash and get something else, but these have stood by me for a long time and I'd rather not replace them if I don't need to. Thanks!