Tuba 36 goes lower than the titan 48 and it is definately a noticeable difference. A Tuba 36 24" wide is not quite as loud as a Titan above 40hz but below 40hz the Tuba shows it's stuff. Tubas and Titans should not be used in singles, especially the slim versions.
I have Titan 48's and Tuba 36's right now in shop. Four Tuba 36 slims in a block are formadable indeed. Titans are just as formadable, but operate in different ranges.
If your music contains a lot of material below 40hz, I would go with Tubas. If above 40hz go with Titans.
Les
EDIT
A PL6.0 is the best amp I have used for Tubas. Run in parallel four can be run off one side of the amp.