Nubyan If you current "Control panel domain" is set to: server.domainname.com and it works fine, then you don't have to change anything there.
Your domainname.com should be added as a regular website to your App+DB role server.
Also when you open the Websites page, your panel subdomain along with other service subdomains (webmail and phpmyadmin) should appear separately from your domainname.com entry, like this:
Just keep in mind that every subdomain from your main domain name should be added as an A record directly on your registrar's environment in order to be usable within your Enhance panel:
@ -> [App Role IP]
www -> [App Role IP]
ctl -> [Panel Role IP]
pma -> [Panel Role IP]
wml -> [Panel Role IP]
mail -> [Mail Role IP]
(translate these to whatever subdomains you use for each service - these are just an example)
