We've used the line converter a number of times as well. Problem is that the HT receiver still needs to be set to "Large" to accomodate both sub and main speakers. This means the client will typically blow the cones out of the front speakers the first time Godzilla stomps or growls - or whatever the beast does (cool movie by the way).

Setting the speakers to "Small" will make for a boomy sub (especially in the back of the room). If you have In/out speaker connections on the sub you're in luck. This is your only crossover to protect the mains.

Otherwise you might try the goofy line out to amp in, amp out to speaker level in. It doesn't have to be a big external amp, just one that will send amplified signal to the speaker level ins so you don't get hum. Then you can set the mains to "small" and the Low level sub out to "On" (without the hum).

By the way, did you try low level jacks on the 16/4's? You might get lucky.