Thanks again, all. I think I'll try it in either my protein skimmer chamber or in the return pump. I like the idea of having the heater after the probes so it's more consistent throughout the system. Having it within 1" of my return and emergency drain just makes me very uncomfortable...especially since the Mrs is not a huge proponent of this new hobby. If something were to go wrong, I'm sure I'd be found stuffed into the tank as fish food.
Since I'm new I'm unsure as to the actual heat output per the radius of the 300w heater; what's considers the "danger zone" and what's not. The return chamber of the reefer 170 isn't massive, if the heater is within an 1" or 2" of the pump do you think there is a chance of it melting the return pump? Albeit I'd rather have that than the drains in the tank.