> Clipping can lead to the amp putting out twice the RMS rated power.
Ok, but aren't there "degrees" of clipping? Check out the attachment; I'm more visual so maybe this will help describe it.

Yes, there are degrees of clipping - from the barely audible to the painfully obvious. Yes, the power increases as clipping increases - 2x rated RMS (given that the stated rated value is honest - which one can't always be sure of) power happens only if the clipping is extreme and the output signal is effectively a square wave (ie effectively no slope at all)
The clipping in "D" is severe but you're still not up to 2x the power.

Am I wrong in assuming that I should increase the amp gain so that 0dB is very loud because: That way, I can properly set the EQ (be diminishing the levels instead of increasing them), which lowers the overall output, and still have a large number of volume increments between "conversationally quiet" and "very too loud"

Nope, that is correct. Just as you say above: I'd increase the gain on the amp to get a wider useful range volume range on the empeg, and get (or build) an attenuator for the other HU. But beware that you probably don't want to use full the full range of the other HUs volume control - most other HUs start to distort their own output at around 75% of the max (remember that the empeg too will distort the output if you go above 0dB on the volume setting, and the signal from the mp3/wav/... is full scale.), so check what volume you have from the other HU at 75% or so after adjusting the amp before getting/making the attenuator.

/Michael