Ok, I see what you’re saying…
Hopefully someone who knows more than I, will post here (I have very little experence with sound reinforcement). For me, anyway, I would need to see the setup; if you are using the main out for the house, does the record out have seperate faders and/or a way to add gain or switch between line/mic out? In other words, is there a way to seperately control the record out? If not, then I think your first instinct was right; you need some sort of pre amp. Again, hopefully someone else can give you a specific answer.