When you have acoustic problems in your mix room and are not sure how your mixes will translate, another way to go (besides stems) is to submit a couple of alternate mixes for songs that you have questions about. For instance, if the lead vocal disappears when you play it in your car, you could submit a second mix with the lead vox up 1.5 dB (arbitrary number) and let the mastering engineer decide which is better.
But any time you submit stems or many alternate mixes, it adds greatly to the cost of the project, since even the "cheapest" decent mastering engineers are charging at least $100 per hour - and usually more like $150.
So here is an alternate plan that is a little unusual, but just might work:
Submit JUST ONE song to a mastering engineer, and let him/her work their magic on it and send it back...plus give you some feedback about your low end management, etc.
Now remix all your other songs so that they sound a lot like the mastered one. Plus follow the tips the engineer gave you. When you are all done, resubmit all of them for finishing touches.
Obviously, you will have to find a mastering engineer willing to work this way, but that shouldn't be too difficult.
