I do ad-free versions of my forum now for a small fee a year, will I be able to do the same with xen and the paid subscriptions?
Yep, you can set up one off or recurring payments for "User Upgrades". Once they've purchased your upgrade, they will be promoted automatically to a new usergroup which can, for example, not display any ads.
On that note, XenForo's template system is awesome.
Firstly, there's special templates (by default, blank templates) that are designed for adverts. So many common placements such as ad_above_top_breadcrumb (Above the top breadcrumb) ad_below_top_breadcrumb (Below the top breadcrumb navigation). There's lots of them for you to choose from, they are all prefixed with ad_
Obviously you can put advert code in whichever template you like, but these are specially designed for the purpose so you just copy and paste your ad code into them.
With regards to hiding the adverts from your paid users, each usergroup will have a group ID. Registered users, for example, are user group ID 2.
If you create a usergroup called "Ad Free Users" and this is where your paid members are put and if we assume that usergroup ID is 5, you can place this in your templates to hide the adverts from these users:
<xen:if is="!{xen:helper ismemberof, $visitor, 5}">
<advert code here>
</xen:if>
That code basically says "if visitor is not member of group ID 5 then show adverts".
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