do u mean notification by topic or by default?
If its by topic its in smf_log_notify
If its by default then its in smf_members
By default... I can't figure out which column it is... I've turned it on and off repeatedly for my account and cannot see which field is being modified. I have all the others, just not this one param

Things you could do if you were sneaky....
Perish forbid!

If you want you can cron a php page to reset everyone to accept notifications whatever they have set....
Zactly...
Also if you were to say add a couple of new fields to the smf_members table to hold something like 'target' and 'board' then you can set another cron that looks for topics in a board and then checks if your 'target' users have replied in the threads and if they havent add a line to the smf_log_notify that means they would get notifications of new replies in threads they may not have even replied to ...
You wouldn't have such an example laying about perhaps, simply for research purposes and all eh?
Of course you could also add some javascript to the post page to set the notify me of replies by default....
Stupidly had not thought of that one yet... but in fact, I could just eliminate the checkbox entirely and put a hidden <input> with the value set to On (like a check...)