Adding a Pre-Out to an integrated tube amp

I've got two sub amps with line level only inputs and a Melody I880 integrated amp with no pre-out. So I want to add a pre-out for the sub amps. Two amps so need L/R not summed to mono. Being a tube amp everything is easily accessible and first thought was take right off from the volume. But then doing some research saw people recommend using an op-amp or resistor, and some others saying you need a schematic to take the signal after the pre-amp stage but before the amp. Seems to me line level should be enough, ie right off the volume will be okay, but will it? Anyone done this and know what will work?