Yes, I understand the concept of impedance mismatch, but never before have I heard of excessive input impedance being a problem like this (although I accept that whacking a 600 ohm mic into a 10K ohm mic input will result in low volume). A thin and brittle sound from a pickup is often caused by putting a 100K ohm pickup into a 10k mic input - but one never had the problem with old valve PA heads the input impedances of which were a megohm or more.

The conventional wisdom was the higher the input impedance (for undersaddle or stick on pickups) the better and indeed the Baggs website specifically asserts that the 10megohm input impedance delivers teh best sound for all pickups.

I don't understand how a high impedance could rob bass or create a thinner sound.