You would need a new buffer circuit or a voltage divider. Assuming the inputs are high impedance (25k or higher) I'd suggest you try 100K in series and then 1k to 5k in parallel. This would give you 1 to 5% of the amplifier output at the sub inputs. You could also use a pot instead of the small value resistor.
Be careful as you certainly can provide too much voltage to the line in and damage it, so use a voltage meter at the small resistor to make sure you stay below 2V
Be careful as you certainly can provide too much voltage to the line in and damage it, so use a voltage meter at the small resistor to make sure you stay below 2V