That's great new that you've found the problem.
Basically, Monolithic is absolutely right.
I can confirm the the 777\767 speakers will never give out the bass you are looking for. Even so, the unit should be loud. In the 80s my cousing had a three-piecer and that was louder than his friends 767, but at the time I still remember the 767 being stupidly loud.
The stock standard speakers are not made for deep bass, they are made for a more punchy\boxey bass that is good for rock. I replaced mine and I've never looked back. If you are will to replace at least the centre speakers, let's just say you won't be dissappointed because all I can say is that the amps are definately putting out those low frequencies.
Also, Stormin points out quite regularly that some of the capacitors may have dried up over the years so that may account for some of the problems.