How clean and at what SPL?
Very few speakers of any size (that I know of) maintain clean output (call it 10% THD) at 20hz at 90 db. If you really want that kind of performance from a small speaker, you'll almost certainly need a subwoofer.
Some people might argue that 30%+ THD is clean enough at 20hz (where hearing is very insensitive). That may be true, but it means that any answer to your question will be awfully subjective (and you'll still probably need a subwoofer).
Very few speakers of any size (that I know of) maintain clean output (call it 10% THD) at 20hz at 90 db. If you really want that kind of performance from a small speaker, you'll almost certainly need a subwoofer.
Some people might argue that 30%+ THD is clean enough at 20hz (where hearing is very insensitive). That may be true, but it means that any answer to your question will be awfully subjective (and you'll still probably need a subwoofer).