Unfortunately that is not really an option in this case. Basically this is a feed through from the main hall - where our sound system is - into the lobby, where these speakers (but not the amp) are. I would like some control over the volume of the speakers in the Lobby from the Lobby.
Is this something that can be placed post amp?
I know of no speaker level volume control that is designed to handle anything close to that wattage. Using anything less is an absolute fire hazard. That heat has to go somewhere and if your transferring it to components that aren't designed to take it they will burn up. Now let's install that in a wall. Bad idea.
The correct way to do this is to remotely control a preamp, as mentioned above. And if you have any possible way to get a single tiny little cat5 cable from the lobby to the amp room, there is no reason to do this any other way. If there is no way to get any wire there, then read on.
This should work fine but is definitely less than ideal. You say that your amp is rated at 425 wpc and unless your diming it on a regular basis, you're probably not getting there that often. Get one of these:http://www.audiocontrol.com/17612/645207/OEM-Integration/LC2i-2ch-Line-Out-Converter-with-AccuBASStm-and-Subwoofer-Control.html
It is around $100. It is a car audio product and will require a 12vdc power supply. Do not get a different one because it's cheaper. For the same reason as above, this one is
designed to work with the wattage you're potentially feeding it. It also has a bunch of "features" you will not need or want to use. This will convert your speaker signal to a line level signal that you can feed a local amplifier of your choosing in the lobby for your lobby speakers from that area. From there you can use the gain control on the amp to adjust the level.
It may be worth a few moments to call Audio Control and discuss some options with them directly. They've been responsive in the past when looking for specialized products.
You will run across people with the best intentions telling you that you can just use a cheap low voltage volume control from "Acme" audio and that your just being too careful. I'm here to tell you there is no such thing as "too careful". Please don't burn down your church. It can and does happen.