And that's unfortunate because it would be nice to enjoy the benefits Bill-k mentions. However, the EP-15A was built without adequate turn-on protection circuitry, and if you turn it on at the 'wrong' point in the 120VAC power cycle with the SP plugged into it in the on position, then the big toroidal transformer in the SP (along with any switched on equipment plugged into the SP) will put too large a current draw on the EP's internal components and fry them.
In fact, even if you have the SP in the 'off' position and all attached equipment to both the EP and SP in the 'off' position as well, fuses will blow (or worse!) as soon as you attempt to turn on the SP
I know it says (even recommends) that you can/should use them together in the manual, but in reality it's a no-no! 2 years ago when I was in touch with him, the new owner of the Exactpower name and patents talked about a bullet-proof, high capacity EP-20A, which because of the patented feed-forward circuitry would blow all other such equipment off the map. So far that hasn't materialized.