To set the default thread subscription:
ACP -> Templates & Styles -> Templates -> Your Theme -> Member Templates -> member_register
Find:
Quote:
<select name="subscriptionmethod" id="subscriptionmethod">
<option value="0" {$no_subscribe_selected}>{$lang->no_auto_subscribe}</option>
<option value="1" {$no_email_subscribe_selected}>{$lang->no_email_subscribe}</option>
<option value="2" {$instant_email_subscribe_selected}>{$lang->instant_email_subscribe}</option>
</select>
Replace With:
Quote:
<select name="subscriptionmethod" id="subscriptionmethod">
<option value="2" {$instant_email_subscribe_selected}>{$lang->instant_email_subscribe}</option>
<option value="1" {$no_email_subscribe_selected}>{$lang->no_email_subscribe}</option>
<option value="0" {$no_subscribe_selected}>{$lang->no_auto_subscribe}</option>
</select>
This only changes it for new members - by changing their default settings. Existing members will have whatever they made it in their preferences.