I don't own a 990 so consider this "generic" advice.

Whenever I hear someone say that they can't get any bass out of a sub (or speaker) and describe the overall sound as you have, I immediately suspect a phase issue of some kind.

I've blown the fuse on a speaker that was out of phase and where I had to adjust its level way up to get any relative output out of it. Of course the speaker itself was, by then, absorbing a ton of power - and luckily the fuse went before any damage was done. It was only when I sat back and asked why did it occur to me to check the leads (which I had recently changed) and found the phase problem.

It's probably not the culprit in your case - but worth checking if you've not done so already.

Jeff Mackwood