By tuner, you mean the digital box itself? I was thinking of swapping-out the box first.
Yes, the digital box. The TV will have nothing to do with how good the reception is if you have an external digital tuner and are just running either composite, S-video or component video signals from it to the TV.
One site to try out is www.tvfool.com which allows you to enter your address (and your friend's) to see which direction the TV stations are.
While some of the information that they give is technical in nature (and therefore of great interest to 'enthusiasts' like me), their charts should be accessible to people who are only moderately tech-savvy... You can use the color code to help figure out whether or not you can expect to receive certain channels based on the type of antenna you have...