It's really a silly idea considering using an in-car amp at home, they aren't that high a quailty anyway - sell it and buy a proper amp with the money.
The volume setting is obviously completely dependent on the gain of the system and the input levels, so 1/3 isn't very meaningful
Incidently - my class-D PA amp incorporates soft limiting - it's 400W per channel, or 200W in my case as I use 8 ohm speakers (15 inch + tweeter PA speakers). Despite this it never seems 'terribly' loud - so I stuck a scope on the output - and this confirms it's providing the correct output levels for a 200W amplifier.
The reason is quite simple, the limiting stops the amp clipping, so the audio is clean at all levels. A non-limited amp would be clipping and distorting, and distorted sound is FAR louder than clean sound - even transistor amps can clip wthout giving objectionable distortion, and this makes them much louder.
So while a small amp may sound loud enough, it's VERY likely that it's actually clipping - which for HiFi use is a bad thing.