That's not necessarily correct in regard to the volume as you've observed anecdotally. Think of the relative electrical resistance in the same way as a water pipe's diameter will effect the flow of water. A larger pipe [less resistance] allows more water to flow, but can the pump [amplifier] cope with the demands required by the smaller pipe [HIGHER resistance] is the question. Hence my comment in Post #10 in regard to the output source.
But hey Mate, if it works ok in the nuvi then it's all good .....