Decor friendly minivan sub solution

[slackerinabox]

The low-end is lacking in our kia sedona (no factory subwoofer installed), any suggestions that won't kill cargo space while filling in the lowend for stock speakers? Underseat subwoofer worthwhile, or would the size of a minivan swallow such a small sub? With our stroller and baby gear I'd hate to get a dual 12" enclosure, but is a single 10" insufficient?