There's nothing particularly "bad" about subwoofer systems that predate Dolby 5.1 surround setups, when properly done with low-pass filters (as in Dignan's case).

Even 5.1 isn't magic -- sure, it works incredibly well when coupled with an input source that was created with 5.1 encoding. But for anything else, it has to do the same tricks as the "old stuff" did when creating a feed for the sub.

Cheers