I've been considering this problem some time for myself. I presently have DVD-Audio & the output from a Quadraphonic preamp (only 4 channels). What I plan to do when I finally get my processor, is use two video switch boxes I bought for $15 each on ebay. They have selection for three sorces, consisting of L/R audio & video. That's 3 channels x 2 units = 6 (5.1) channels. The only thing not convienient is having to push two switches instead of one.
Other options: the Sony preamp mentioned above (that just came out this year - I can't believe it's been discontinued already).
The top of the line Denon reciever has two 5.1 channel inputs.
Or sell your dvd-Audio & SACD players and get the all-in-one Pioneer DV-47a (my plan - to get SACD)