Can a sub be hooked up to speaker posts?

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.

One thought that occurred to me is if your amp is a fully balanced design, then you would NOT want to connect your sub this way (contrary to my previous statement). In this case you need to connect the Sub ground lead to a chassis ground point and not the - terminal of your speaker outputs.

However if your amp is a plain vanilla unbalanced design, then this is simply a question of which end of the speaker cable you connect the sub cable to. While the speaker cable introduces impedance, I believe it's negligible as far as when the sub sees. So to answer your question, it can be done, but you'd only know if you like the results if you compare it to connecting directly to the amp terminals.

I'm thinking of doing the same thing. When hooked up this way with rels how much of a load will be put on the amp. I'm concerned with distortion when I crank it up. Is it simply to get the same signal as the speaker then amplify it. What I am saying is, is the amp active when connected this way or only for the LFE input.