In general, I would prefer a separate high powered subwoofer because it gives you more placement options, allows the subwoofer to be positioned to optimize the bass for your room, and reduces the strain on your AVR by allowing it to cut off at ~80Hz. I realize this might not be ideal for all folks, so maybe a powered tower (upgraded 533-PT) along with an upgrade for the 633-T. With that in mind:
(The tweeter isn't variable here, so it's not an option)
Why not? Even if you want to stick with the silk dome, at least the option for one or two?
1. midrange driver: one 4", two 4", one 5.25", two 5.25", one 6.5" or two 6.5".
This is really tough to answer solely based on count and size. I would expect a lot of listening/instrument tests to best match and balance the entire system. With that said I would start with two 5.25”
2. Bass driver (optional): one 5.25", two 5.25", one 6.5", two 6.5", one 8", or one 10".
With a separate subwoofer in mind, I would start with two 6.5”
3. Subwoofer (optional): 8", 10", 12" (If you choose one of these tell me if it should be powered or not. If it is powered, how many watts do you want?)
As previously mentioned, I prefer a separate high powered subwoofer (I would like to see an upgrade for the S-12)
4. Port. Front, rear, or none?
Front port since it gives you more placement options. I like the way you did the 633, simply hide it behind the grill.
If you want to go another step, tell me what type of dimensions you prefer. Have some fun!
Since I would prefer it to blend into the room rather than stick out on its own, I would keep the height less than 48”, the width less than 15”, and the depth less than 20”. (as if something with those dimensions won’t stick out, its all relative)