This mod and use of the phone input is for driving sub woofers only
I'm sure most of you old timers have a 2ch pre amp getting dusty. I'm using a HK 725 which was sota in its day. The phone input uses the RIAA equalization which is a great boost for a sub. You will need to put a 1meg in line so not to overload the this very high gain input, I recommend only to install the 1meg resistor inside the pre amp's metal caseing as shielding is of the highest importance
. At frist, just test it out at low levels to hear if there is any input overload. This will clean up any muddy sounding subs. Please take into account that demands on the sub will be higher than before, not for wimpy sub drivers and underpower amps. Once you try it, you'll love it.
Yea, I know that there is a roll-off starting at around 20hz, but that shouldn't be a problem for most, it will help to lessen the demand on the hardware.