Not sure how to set the default but you can can change individual notifications by:
Open this link ->
http://www.the370z.com/subscription....1&folderid=all
Check the subs you want to change (box on right side of listing lines)
Use drop-down menu at bottom of list to change settings.
Edit: To change default, click on "User CP" in nav bar at top of page. Click on "Edit Options" in menu on left side of page. Scroll down a little bit to find "Default Thread Subscription Mode". Use drop-down menu to change setting.