Is the transducer self-powered? Are you running a 5.1, 6.1 or 7.1 set-up (ie. are you using all of the 770's channels? Do you still have the old audiosource amp?)

Here's some possible solutions depending on some of the answers to those questions.

Split the LFE/sub output. If the transducer is self-powered connect it directly to one of the split signals. If it needs its own amp, use the audiosource. If you're doing 5.1 or 6.1 use one of the spare 770 channels instead.

Run the other split to the sub. Then re-calibrate the sub level in case it has changed due to the split. Presumably there's a gain control on the transducer as well. Adjust it to suit / match.

Then again I could be wrong.

Jeff Mackwood