I don't have a lot of Soundstream experience, but the term "single-ended" means "NOT balanced"...
I would meter from the L outer shield of the RCA connector to the R outer shield. If the meter shows continuity, don't try it.
I can tell you that true balanced inputs like the JL Audio, the Tru Technology, the Arc Audio, and others, work on the HU outputs, as well as the Zapco Symbilink inputs. The Alpine PDX seem to work, and Alpine does not claim balanced. The DLS amps work on the Acura balanced, and they are not true balanced either - but if you meter the shields, they are not common.