georgelofi has the best low cost solution.
" What is likely causing too much system gain? This gear has been used together before (not the MMGs but other Maggies) and I don't recall the problem."
Its not that any one component has too much gain as a design flaw, you just have to be careful when you match components. In your case, the 3.6's are a much more difficult load to drive than the MMG's. Your line level amps needed to deliver a lot more power to your power amp to get the same volume out of the 3.6's than your MMG's. This makes the volume control much more sensitive.
It's also worth noting that there is more than one way to fix this issue. If you know what you're doing, swapping out any one component can fix it. For you, that's not a practical solution, so that's why a passive volume control was recommended. In your case, this can also lead to a substantial increase in sound quality by eliminating your active line stage (the GTP-500). Doing this should help with whatever brightness issue you have, that still remains after the speaker are broken in.