Design of Low Pass Filter
I think that what you are saying is to "add gain" but not "invert" the signal. Your added amplifier is an "inverting" amplifier. You are "going into"  the  -   input. So just use a "non-inverting" amplifier, "going into"   the  +  input.
Not sure of your experience level .... do you know how to do that? You can search for "non-inverting op amp amplifier" on a search engine. Otherwise, just ask and I or others will draw it for you !