The further the bass has to travel the louder it sounds thats the idea of facing them away from you. You can stand right up against a heavy hitting 12" and not feel much, the moment you step away a few feet WHAM! Depending on the power of your set up and the freedom the sound has to reach you your techs may be right. If this is a benz facing the subs against the backseat would muffle the sound because of the gas tank. I would face it away to have the bass build enough 'momentum' to get through the parcel shelf especially if my system isn't that strong. Just my humble opinion I am sure you'll get some other responses from some very capable people here as well......
There are other variables to consider like is this a bandpass, free air,sealed...etc