Hi there...I have a Canon S400 and housing and a Sea&Sea strobe. Originally I bought the Sea&Sea DX tray and arm, but have recently run into a problem with it. I now want to add the Inon AD lens adaptor, that allow me to add macro/wide angle lenses/filters to the camera and it will not work with this tray. So I sent an email to ULCS to see if the Inon lens adaptor would work with their digital tray. At first they did not know, as they were not aware of the Inon adaptor, but after taking a looking and talking with Ryan at Underwatercamerapros.com, they worked out that you can use the ULCS digital tray with or without the Inon adaptor. Whilst I have not yet got around to buying it, this is what I am planning on doing. The ULCS digital tray is a very good one, with a nice handle and the ability to take most arms on top of it. My plan is to use my loc-line arm on top of the handle, to give me the flexibility of positioning. I was also contemplating an additional piece to allow me to attach a UK Q40 dive torch as a focus light.
As I say, I have not got around to buying this new setup, but I have spent a fair amount of time researching it and feel that it is the best way to go for me. ULCS have been incredibly helpful and if you are in the US, Ryan at Underwatercamerapros.com should be able to sort out both the tray, arm and lens adaptor for your S400. I will post a pic of my setup once I have bought it (probably at the London Dive Show in March).
Hope this helps