Forget the Y-cable, that's doesn't sound like the problem. It sounds like for some reason, you're not getting enough signal to the sub. There are a whole bunch of possible solutions to your issue so here is a short list of things to check:

1) What are your crossover settings in your 990?
2) Are your speakers set to small or large?
3) What kind of demo material are you using where you're expecting a large sound from the subwoofer?

Those are good starting points. I would also advise that you check out the 990 saloon and look at all the posts about bass management. If you don't have your settings correct, your subwoofer won't work the way you expect it to. Let us know how your tests go.