Dear @erik_squires : I’m not against what you already achieved because at the end is what you like and what you think.
I added subs to my really full range speakers for one main reason that name is: lower IMD. My ADS per sé goes down to 16hz but you have to pay a very high price for that: high IMD.
You have a 2-way speaker designs and this could means that you crossover 2.5khz to 3.5khz ( more or less ) from the woofer to the tweeter.
In any two way speaker design the IMD is rally high and this is the reason why you in reality need a pair of subs: to lower dramatically the IMD and when you do that the " ligth " comes out and your room/system will shines as ever before and as a side advantage is that now you can go deeper and with better quality performance in your bass range that’s where the MUSIC foundation lives and obviously that the electronics of your main speakers will works with out any kind of distress and with higher headroom and you don't need to measure anything because the best " measure " to that IMD are the IMD comparison you make trhough listening tests before and after. You can tame the IMD but you can't disappeared by room treatment or any other way because the IMD is developed by the speaker 2-way woofer.
Imagine your today IMD levels in your speakers when the movements on the woofer reproducing 35hz-40hz at the same time needs to reproduce frequency range above to at least 1khz ! ! ? ? ?
Tha’s my take for subs In any passive speakers.
Regards and enjoy the MUSIC NOT DISTORTIONS,
R.