The heatsinks on these amps are designed with convection cooling in mind. They work best when mounted vertically so that as the air absorbs heat from the sink it can easily rise and draw cool air in at the bottom of the sink.
You can mount a plate amp horizontally but if its driving a difficult (low impedence) load then you may have thermal shutdown issues. You could mount a fan to the box to force air across the heatsink.
I can't quite picture what you're describing in terms of the amp box serving as a stand for the sub. It sounds like you want to use the sub box as sort of a large baseplate for the sub and the plate amp facing upwards. If that's the case then think about how tall the legs will need to be in order to give you decent access to the plate amp inputs and controls. Seems like it would be really awkward to access the amp. But then again I'm probably just not understanding your description.
Thanks Ryan, actually you interpreted my description perfectly...I thought something like a 4" gap would be enough for the power lead and LFE feed, plus the very occasional access to phase and the auto on or off switch. The other controls would be on the remote controlled remote plate mounted at the front. I was kind of thinking that if the driver was facing downwards at just 4" away it might waft a nice breeze over the amp too, but maybe not.
I think I understand your description. Ryan is correct in that most amps operate best, thermally, if the fins are oriented vertically. That said, I very much doubt you would have a problem in your design, given that the fins are still above the output devices and in open air. I doubt the woofer air movement will be a noticeable advantage, but certainly can't hurt. In my experience, plate amps (at least on commercial subwoofers) do not get very warm at all. In fact, one I built just last week, after quite some time of driving it near reference levels, is barely warm, and I do mean barely. Now this is only a 125w @ 4 ohms amp (and that's the impedance woofer I was using), so it may be that the 250w+ amps common these days, run a lot hotter.
I agree that 4" should be enough for the small cabling and limited access. After all, unless you are constantly tweaking and testing, how many people actually touch their plate amps? Unless your front end equipment is in the stone age (by that I mean > 5 years old ), you won't need the crossover, nor the level control, and how many people are constantly fiddling with the phase switch, honestly? I have zero controls on my subwoofer amp and I don't wish I had any.
True you won't access the amp controls much. But I know that when I was tweaking phase I tried about 8 different positions before I found the best spot. I would really suck to change the level knob accidentally and hose up the rest of your calibration work.
The Parts Express/Adire 250 watt amps do tend to run hot when driving 4 ohm loads. I know this from experience with the PE 300-794 and several others have posted similar experiences. A few guys have seen thermal shutdown issues when driving especially hard loads or perhaps they just had an extra-hot running amp.
By comparison, the plate amp on my SVS 25-31PC (not PCi) runs much cooler.