Or is there?

In terms of using the sub, it's a little more complicated.
A 5.1 sound track contains five full bandwidth channels (3Hz to 20kHz if I remember right) plus one limited bandwidth LFE channel (3Hz - 120Hz). Since pretty much no one has full range speakers and sound is effectively impossible to localise at 120Hz or lower, an AV Processor will redirect not only the LFE channel but also additional information from the other channels to the sub. The cross over where this happens is set in the Processor and can be changed to cater for surround speakers which can cope with varying depths - but for example, THX say that this should be set to 80Hz when using THX certified speakers. This is not a hard cut, a carefully calculated function curve is used - incidentally, this function usually means that sealed speakers and subs actually integrate better.

So basically your sub would play the entire LFE channel, plus whatever is redirected from the other channels, whether that's two or five or seven or whatever.

The benefits of this are quite a few, the main ones being:
- you'll actually hear the bass from the surround channels, since most speakers would be incapable of playing much below 40-80Hz anyway
- less load on the Processor's amps, as bass usually requires more air to be shifted; now that it's redirected to the sub it has an easier job
- your surround channels will probably sound cleaner since they're not struggling to render those lows

With stereo music, there's no LFE channels, so Processors will do the same as above and apply a crossover and redirect to the sub below that crossover frequency. However, to appease the purists, most manufacturers also include a direct mode which won't apply any crossovers or bass redirection, and often none of the room correction algorithms either. On Yamaha that's called Pure Direct, and it will just use whatever channels your original source material has. Obviously if the source material has a .1 channel then that will still go to the sub.

Now, if you do that but you still want to hook up the sub, firstly you need a sub with high level (amplified, or speaker level) inputs. You then hook up cables from your L and R speakers to these, and manually set the high level crossover on the sub. It's a game of trial and error in this case, though it should be near the speakers' low -3dB output. You might also need to play with the phase setting on the sub to better match what's reaching your listening position.

There are subs which can accept pre-amp (non amplified) inputs for both channels as well, it all depends on your gear. If you tell me what the speakers and sub are I'm happy to help.

As you can imagine, it gets a little messy if you have both the normal LFE connected between the sub and the Processor and also high level inputs from the speakers to the sub...

Thanks dude. I knew I'd get a considered response from someone at some point! <glares at Xen>.
  ^This. That's the situation I could have. How bad does 'messy' sound? I have these (in maple) (with added what hifi stars :/ ):


They're Acoustic Energy Aegis Evo 3s. The sub has the high level inputs. Manual's here.

I haven't read the rest of the thread but my Denon amp has a +main in the speaker setup which shoves some of that bass down to the subwoofer from the front speakers as well as the LFE track.

How's it cabled up?

Nice speaks.

NAS to Pi, Pi to HDMI splitter, splitter to TV/amp. Amp to speakers and pre out for sub.

Just bread Serg's post. Now I'm not really sure what +main is for as the subwoofer will get some of the bass from the speakers below the crossover anyway. I just have it set to normal.

I think the +main is for when the speakers are set to Large and the lower end is still sent to the sub.

I have to stay out of this thread. I am an audiophile wannabe and I can't afford it.  :'-( 

I still need to pick up a reasonable 5.1 set. I'm currently working with 5.0. Or I just get a sub.