SPL meter results relative to Magnepan's (supposed?) craving for power/current

Hi all...
My thoughts are; if my ambient/background level is about 40dB and readings of "music" at my typical SPL from my listening position show about 75dB, that means that what I'm actually hearing is mostly noise and not my Maggies? Surely yes... Right? 

Even though my Maggies are in a very big room, do I need to rethink my amps power output of 400 WPC if I prefer a lower listening level? Overkill? I understand the benefits of "headroom" and amp/speaker "synergy".

At which SPL do your Maggies wake up and sing?
Thoughts? Comments?

Thanks!