Pre out is the way to go. The differences are significant. I have done both in my system.
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Simonleemd, does your sub have an internal X-over with inputs and outputs? Though not ideal, unless you have a pre out, it's probably better to run your fronts through the sub. The output signal from the sub should have the lower end freq that the sub is using removed. Thus leaving your speakers to handle only the mids on up. Otherwise, if you run the sub off the #2 spker posts, it could result in an over emphasis of the lower end, since you'd be doubling up on the low freq with both your speakers and sub.
I agree with warrenh: you want a sub pre-out. Otherwise, using internal crossover of a sub (if it has one) requires you to run your main left and right signals through it (just what you don't want to do is run your main signals through more electronics, especially since the sub electronics will almost never be equal quality to your main preamp or amp). You can run a separate run from those second binding posts you mentioned, a sub's internal amp is specifically matched to the sub speaker, usually providing lots of short term amps. I would guess that your regular amp would do a much poorer job of feeding the sub speaker directly (while also supplying the rest of the drivers in your main speakers). Do not connect the second binding posts of the amp to the line-in connection of your sub. That would be a horrible impedance mismatch and might blow up the sub's internal amp.
If you have not bought the preamp yet, why bother to buy one that has an outdated design? Almost all current preamp/processors provide for a sub out.