Originally Posted by BigLouis1971
I thought about having two sub-woofers eventually. I was thinking to add one to the high level outputs of the receiver to extend the Polks frequency range and try to make them sound like a bigger tower ... The second sub can be hooked to the sub preamp out of the receiver to provide whatever bass comes from that additional channel.
Is there a difference between what I just said and use a splitter to hook up both subs to the sub out of the receiver? Since I'm no expert all I can do is guess. I guess that since the first sub is actually hooked to the high level output of the receiver, that sub will enhance the bass that's supposed to come from the front/right channels. The one connected to the sub out will provide whatever bass was added by the filmmakers to that channel and will be a different bass channel.
The great thing about using the AVR (receiver) sub output is that you can configure it for true bass management - you configure the front, center, and surround speakers as "small" with say a 80Hz crossover frequency. The AVR will strip all the low bass below 80Hz out of the signal being fed to all those speakers so now they don't have to strain to produce lows at the very bottom of their range (the most difficult frequency for them to reproduce) and at the same time this frees up a lot of amplifier power (those low frequencies take the most power) for the mid-bass and mids. This allows your other speakers to sound better.
The sub will then get this bass below 80Hz from the other 5 speakers (along with the LFE information added to movie soundtracks) It is much better at reproducing those low frequencies because that is what it is designed to do. So your other 5 speakers will sound better in the highs and the mids and your bass will sound better coming from the sub - this is the magic of bass management. As you said, the bookshelf speakers with a sub will sound like (or even better) than tower speakers without a sub.
The second advantage of a sub (and even greater advantage of multiple subs) is that it can be placed in better "bass friendly" locations. It is almost never true that the best place in the room for bass is the same location as your main speakers, so it is a huge advantage to be able to move the subs where they give the best in-room bass response for your room. As others mentioned, when you have two subs in more ideal locations (and EQ them), you get even better and smoother in-room bass response.
Finally, to answer your questions, you want both subs reproducing the exact same bass so that it is more even throughout the room - this makes the other 5 speakers sound better and gives you the best LFE by giving you the best, most uniform bass in your room.