You can categorize your AWeber subscribers using the "ad tracking" feature.
Many customers use this feature to keep track of where they acquired each subscriber. Valid ad names may contain letters, numbers, hyphens, or underscores, but not spaces. Ad Tracking values will only use lower case letters.
You could assign each subscriber that comes through your web form the ad tracking category "web_form", those that come through newsletters the category "newsletter", and those that come through classified ads the category "classified".
Once you've used this feature to track your subscribers, you can go to your Reports page to compare the responses to each ad.