I'm not familiar with your Symphony, but I'm going to guess that it is wired the same way as the H8000, so that when you connect them together with a "one-to-one" cable, you are connecting in to in and out to out, with predictable (non) results. You would need a "crossover" cable, but these are not widely available.
If you are handy enough to make your own cables you will know what to do, otherwise we recommend a "snake", with DB-25 to XLR. These are available from multiple sources, including (of course) the Eventide Web Store. You can either use one of these to give you XLR AES ins and outs, or two to connect DB-25 to DB-25, with a useful insert capability.
Firewire would be a cheaper option, but my guess is that your unit expects to connect to a computer, rather than a box like the H8000.
