I cross the Heil over at 3kHz and you can use that portion of the schematic if that is your goal. This assumes the use of the "low impedience" Heil. We run the "Heil load" at ~8 ohms with a 4.7 mfd cap so your estimate of 9 -15 is pretty good for a 4 ohm load. The Heil does not need the additional volume gained by removing the resistors/Lpad and is plenty loud with 4.4 ohms added to it.