In the documentation from PAC, it says you need a "Cable A" (from your head unit) and a "Cable B" (to your amplifier). What exactly are these? Are they cables you need to purchase separately?

It's all part of the package. The OEM-1 comes in three parts:

1) The cable that connects to your aftermarket stereo. ("A")
2) The OEM-1 unit itself.
3) The cable that connects to your car's factory wiring harness. ("B")

They are a set. You buy them all together all at once. You have to specify which A and B cables you need at the time of the purchase. For instance, when you order them from www.tune-town.com, their online order form gives you drop-down lists to select the necessary cable bits.

Since all factory wiring harnesses are different, and since many aftermarket car stereos are different, this is the easiest way to make it completely "plug and play". They designed them so that everything snaps in.

In the case of the Empeg, the "A" cable should be the generic RCA cable (I believe its part number is "RAD-RCA". The "B" cable should be the right one for your brand of car.

There's a photo of it here.