Changing users' email notification preferences

I have created 300 new users using a bulk CSV user import plug-in.
All is well and good.
The only problem is, by default, the “Accept to receive post or page notification e-mails” and “Accept to receive e-mails sent to multiple recipients” options are untagged in the “E-Mail Preferences” section of each user profile.
As an administrator, how can I bulk-activate these options for every user? Of course, I could go through my whole user database, looking on as each person in turn, and do it from within their profile page. But life is short and I am not getting any younger.
Any help, advice or guidance would be hugely appreciated.
Many thanks