The cheap solution is "Looping in" a receiver into an "auxilary or bypass/tape/monitor input", which would power the rest of your speakers, and add processing.
HOWEVER, for simple improved movie sound from your existing system, I'd simply add a Digital process to your mix, looping that in instead....thus keeping the system 2 channel. You can then add a necessary sub, and get all the processing you NEED for DD/DTS. You can't get the same dynamic sound simply processing in a DVD player, etc...it's not as good...and you need a processor of some sort for movies via DD/DTS.
HOWEVER, for simple improved movie sound from your existing system, I'd simply add a Digital process to your mix, looping that in instead....thus keeping the system 2 channel. You can then add a necessary sub, and get all the processing you NEED for DD/DTS. You can't get the same dynamic sound simply processing in a DVD player, etc...it's not as good...and you need a processor of some sort for movies via DD/DTS.