For an inexpensive ch 13 attenuator, get one of these:
http://www.channelmasterstore.com/JO..._p/cm-0583.htm Put a terminator on the ch 13 input and run the signal trough the Jointenna.
You also have a very strong UHF 14 coming in off the back of the antenna. If you can find one, a Channel Plus NF-469 will cool it off a lot.
The gain control on that amp is irrelevant. It's only for the active return path which is not used in an OTA application. It's actually a drop or distribution amp intended for cable TV applications, it's not a preamp.