If you're not using the speaker outputs, then you don't need the STK4151 - you could simply remove it.
From your voltage readings the new STK looks to be faulty, those chips are pretty well long obsolete, and any for sale could well be fakes - pin 10 should be zero, and it's not. A minor mistake in fitting it (should as a solder short across two pins) could have instantly killed the chip as well.
Take the chip out, and use it without, just as a preamp (as you do anyway).