You can bridge each side but you can't turn it into a mono amp. I have seen guys in the past bridge each side and run one side to each voice coil on a dvc sub. Would be best with 2 subs imo, as its not advised to run 2 different channels into 1 speaker. But I have seen it done.
The old blue ones from back when Soundstream was a premium brand? I'd advise against. That would be a fine amp for mids and highs (though it's probably nearing the end of it's life or the point where you'll need to have some components replaced to keep it going) but with the cost size and efficiency of modern monoblock amplifiers there's absolutely no reason to over-work one of those old dinosaurs on sub duty.