I'd say generally anything with an amp hooked up...since most HU's don't put out more than 15-20watts rms, I would say most amps out now are 30watts+. HU power was loud enough for me - I didn't want distortion though
I voted 501+, but my answer pretty well jives with the comment above. If you have to upgrade the electrics, alt especially, for daily listening then I would call it high power.