SDDS has 2 extra front channels: One between the Left and the Centre, and one between the Right and Centre channels. It is usually listed like this: L, Lc, C, Rc, R.
Obviously cinemas needs to extra have the extra speakers to be able to take advantage of these extra channels properly.
This is just a guess, but the reason why the levels may have been loud in the screening you attended, could be because that if the cinema you were in did not have the extra speakers, the Lc and Rc channels could of been routed to the existing Left, Right, and Centre channels instead. The overall levels of the front sound stage should then be lowered because of the extra infomation being routed to the L, C, R chanels than is with DD and dts. Your cinema may or may not have done this, which in my experience it is a common mistake to make. The other reason could be that the Lc, Rc channels were not used and the remaining L, C, R chanels were raised in level to compensate.
It is very unlikely that you will ever hear SDDS at home, probably because that SDDS as a cinema format is losing out badly to DD and dts. One of the reasons is because of the extra channels that some cinemas cannot take advantage of. Another reason is that the SDDS sound track is located on the very outside edge of 35mm film, the edges of 35mm film is the first part to wear out so the SDDS sound track is the first to suffer, which can be as little as only 20 screenings before the SDDS sound track is unusable or of a poor quality.
I'm sure there is a SDDS web site run by Sony which has more info, but I can't remember the address at the moment!.
Cheers.