I can see two different paths.. And Different threads are cheap...
If you are integrating a USB sound solution and class D amplifier this makes it much easier to accomplish things on a PC. I would strongly suggest using a DAC solution instead of a simple sound card solution. The cost difference shouldn't be much but the flexibility would be significantly more. Personally I think you should stick with the USB solution even if you tie it to an ARM unit to make a self contained unit. ARM units have USB ports too... Make it a true dual purpose unit. With a DAC unit build in instead of a generic sound card you can control what frequencies and any delays one might need for each output. Would be an awesome setup.
If you have a NDA on the setup you can likely sell the unit but can't disclose the programming aspects of it. You could contact the company about your intentions. Generally the population is able to order small amounts as "development" items.
Another option you could look at is having an expandable module setup. On the master unit you can have the dac/sound card and setup a data bus and power bus. Then you set it up to have as many modules as one could hook to it. Probably with a design power limitation. (So maybe 3 2 channel amplifiers and 2 1 channel amplifiers might be the limit... ) This would allow for easy expansion and if someone just wants a 2 channel amp they can do so or if they want 6 channels of stereo and 2 of subs they can do that as well. You might even release a module that has 2 4 volt audio outputs on it with an amp turn on so someone could add a subwoofer amplifier of their own and still benefit from the other channels. I think having plug in modules works because you could release it with a 4 x 45 watt amplifier and if someone wanted a 4 x 75watt instead they could go that route or could add a sub amp on it if they so desire. Locking yourself into one design forces you to have to have more than one variation if you want to be able to power more than one setup.
I think it is a good idea just depends on how you want to make it work.