Trying to setup a dev subdomain, but url loads as /dev/ instead

Sounds like it is something left in your database. Try to make a new backup and import it again then run this SQL in your MySQL.

Input your old domain as http://www.old-domain.com and the new in http://www.new-domain.com, also if you have changed the prefix (wp_) to something other, change it before run.

UPDATE wp_options SET option_value = replace(option_value, 'http://www.old-domain.com', 'http://www.new-domain.com') WHERE option_name="home" OR option_name="siteurl";

UPDATE wp_posts SET guid = replace(guid, 'http://www.old-domain.com','http://www.new-domain.com');

UPDATE wp_posts SET post_content = replace(post_content, 'http://www.old-domain.com', 'http://www.new-domain.com');

This SQL changes all possible url:s from the old domain to the new one.

And you have not forgot to change the database in wp-config.php file?