Add a pot to make it infinitely adjustable for actual gain. Change the 22k to 2.2k and change the 3.3k for the outside lugs of a 500ohm pot with the wiper to the output jack. This will allow you to dial in a stronger signal at lower gain settings and down to no signal for higher gain settings. No need to ever lift the circuit because the impedance is so high compared to the speaker load that it has virtually no affect on the output impedance.
EDIT: Re reading your post there's a very easy answer... If you're happy with the performance now, just leave it the same and keep the line out circuit connected to the 8 ohm tap. The voltage will remain the same as what you have now. The line out circuit operates at the tap voltage it's wired to regardless of which tap the speakers are using.