Blaupunkt THA 475 PnP Amp: Multi-channel
Check the link above. Blaupunkt offers a plug and play amp to boost the amp to the rest of the factory components. It even give you some RCA inputs for an Ipod. The one above is a 4X75Watt amp. They make a 5 channel unit as well. Then you can get the better output with serious wattage and upgrade your speakers as you need and keep the factory head unit. I have a convert and am really short on a place for a sub, but if I can get enough watts through a decent pair of speakers, I can get the music quality I'm looking for. I've seen these units for about $250 at the cheapest. Make sure you get the units that say pnp - it stands for plug and play. They make similar units that are not plug and play for about $150 -- but there is a big difference convenience.
I'm seriously considering doing this. If anyone beats me to it, send me a note.