I have been searching for the same thing. I have two subwoofers that are good quality but are passive. High quality receivers almost never send a speaker level signal to the sub. They always send a preamp signal for an amplified subwoofer. However, most Home Theater Systems (HTIB) have receivers that send an amplified speaker level signal to their subwoofers. You can't purchase those receivers standalone, but maybe you could get one used on craigslist for cheap and discard the low quality speakers. Most HTIB receivers will not have a lot of fancy features like you would expect on a $200-$800 receiver, but some of the higher end ones do have a lot of connections at least. For me personally, I've decided to just use a good receiver and then use a standalone amp for the subwoofer, which after much research, I've found a few on ebay for as cheap as $17-$25. Just browse the following section, sorted by low price, and make sure you buy one that includes an AC adapter for use in the home.
Ebay > Electronics > Home Audio > Amplifiers > Power Amplifiers > 2-Channel
Most of them take RCA inputs and speaker wire outputs. For a subwoofer you would only use one channel.