I don't give one iota about "paper watts", "paper impedance", a-weighted THD, or any of the other stuff you read on a spec sheet. Never have...
What's important, to me at least, is if the amp 1. Sounds good, 2. is reliable, 3. isn't noisy, and 4. fits in my trunk. I've never gone out and bought subwoofers and then tried to find an amp to work with them. I've always bought an amp, or amps, and designed the system & install around those taking care to not require additional batteries, an HO alternator, or who know what else.
Amps designed to run at those low impedances usually do not sound good and none are reliable. When you ask a switching device to do twice the power you halve it's lifespan. Furthermore, many users choke the power supplies b/c they do not supply enough current to their new monstrosity. What's the point of getting a cheap $200 3kW amp if you need $500 worth of batteries, alternators, and 1/0 to not kill it? To me, that defeats the purpose of a budget system especially since you can get just as loud and sound just as good, if not better, with smaller amps & different drivers.
Honestly, I do not see why your garden variety street system would ever need more than 500w. There's plenty of good sounding speakers out there that are quite efficient...it's the subwoofers that are the problem as they've become quite fat over the years. Remember when kids used to be able to terrorize the neighborhood with a pair of W2's & a Punch60? That doesn't happen now. Remember when $300 for a 12" sub was really expensive? What the heck happened?
You do not need to spend a lot of money to sound good nor do you have to spend a lot of $$$ to get loud. You need to plan everything accordingly and in my experience, building around the amp is the smarter way to go about things.