Can a sub be hooked up to speaker posts?

This is the arrangement for the REL sub I have, but most people want to use the sub's crossover. In this case, you have to run the amp output to the high level input on the sub, and then out from the sub to your speakers. In the case of amp>speakers>sub, you would only use the sub for very low frequencies, to augment the full-range speakers and use the sub's crossover to limit the upper frequencies from the sub. I need to know more about your system to answer your question.

I also have REL and run it off the speaker outputs, which REL recommends. But as Tgrisham says, we don't know what sub you are using. If it is one that can do it , then listen and see which way you like better. It will largely depend on the quality of your main speakers and the interaction of your system with your room.

So, instead of running the sub from the amp, it would be amp, to speaker to sub.

it's implied in your statement you're using hi level connections for your sub, so yes, this is the same as connecting to the amp, except you have a little section of speaker cable in the middle.

I just looked into this with help from several of these same posters.

As Stanwal noted... this is the Rel subwoofer solution. They provide a special cable to hook the sub to the mains.
they design their sub so that is very high impedance. which I understand to mean that your power amp will not be loaded up with this kind of connection.

However, I don't think most subwoofers are designed this way and I would be very careful to wire it up this way. I would differ with Bdgregory and suggest you contact your sub manufacturer just to be sure. (not an expert on this one)

I do know that the standard way to use high level inputs is to go Amp to Sub to Main speakers and the Sub uses a fixed highpass filter to send the upper frequencies to your mains and then a low pass cut off to select the sub cutoff frequency. As Stanwall suggested I would... I did not like the effect of going through the sub's high pass filter in my system.