In reply to:
If the speakers can play down to 35Hz why not let it play it?
First you have to realize not all the low bass is in the LFE channel, many times there will be low bass (as low as 20hz) mixed into the main channels too. So if your towers are set to large and they only go down to 35hz you're missing out on that bass. I just cringe when I hear people talking about setting their VP150's or bookshelves to large, because they're definitely missing out on bass that is there in those channels.
Second, you have to realize that the crossover is not a "brick wall", but actually a slope. In fact, the ideal recommendation is that your speakers should be able to play a full octave below the crossover point, since they will still be receiving a signal at those frequencies. That means that for a tower that can play down to 35hz the ideal crossover would be 70hz. Your speaker is still getting a signal well below 70hz, it's just rolled-off as you get lower. So it's not like you're wasting the capabilities of your speakers.
The reason a 80hz x-over is recommended is because even though 70 might be ideal for your towers, it's probably too low for the other speakers in your system. Since most receivers and pre/pros don't support multiple x-over frequencies, it's best to just cross everything over at 80hz. This probably means you're breaking the octave rule mentioned above for your center/surround speakers, but you really don't want to have your x-over any higher than 80hz because bass will start to become directional as you go higher.
