SDP and surround modes

I had (have) the same problem. It didn't seem to have any particular cause or reason why it would start or stop. I got tired of this & purchased a refurb Onkyo (shoponkyo.com). As long as I felt I had nothing to lose I took the cover off the KLH & started pulling & reseating the remomable plugs. When that didn't help I started banging different components. When I hit a black semiconductor on the right side I heard a pop, like when I would lose the surround. It is labeled "MX" just along side there is a paper label(0270100). It is the largest black semiconductor on the main board.

I then banged it a bit harder a few more times & the surround was back & worked fine. a few days later I lost the surround again. A few knocks on the "MX" brought in back again. I can't tell for sure if this can be pulled & reset, but for now I am leaving the cover of & am ready to give "MX" a couple of shots with a screwdriver handle.