Unfortunately, older AudioSuite plug-ins won't work with Pro Tools 5.3. The main reason is that the ways in which plug-ins, DAE, and the Pro Tools application communicate with one another has changed. DAE and DSI are now shared libraries (rather than a separate application for DAE, and an INIT for DSI).This was done for several architectural reasons, mostly due to requirements that will appear with Mac OS X (in a future release of Pro Tools). Since we needed to break backwards-compatibility with the TDM plug-ins in Pro Tools 5.3 (because of the major hardware changes on the HD cards), we decided to get the "glue" changes over with at the same time. We really hate breaking backwards compatibility (and we know our customers/partners hate it too!), so we decided to plan ahead and bite the bullet now, rather than risk another break in the future. It seemed best to consolidate all the changes at one time. This will make it easier for our plug-in developers when we release a Mac OS X-compatible version of Pro Tools.