Kicking this old thread. My box has the *exact* same issue as yours with the working alarm. Pops on switching the stereo lever, but the preamp controls don't do anything at all. And it also has the backgrounde noise plugged in. Mine's a Helix by the way.
I am still investigating it, thread is in the restauration section (fixing ebay legend). So far I have concluded that both my amp chips work OK. The function switch works OK (I traced the audio signal). It does indeed seem to be an issue with power to the preamp board, or at least that's my current hunch.
The power distribution on this thing is really weird. Usually there's a separate print with the transformer on it to drop voltage, a diode bridge to create DC from AC, and a couple of big caps to smooth out the power. Well, there is such a print, but it's missing the filter caps. The output goes to the pcb with the power and alarm buttons on it. From there, multiple wires go to multiple boards. I *think* the main power line goes to the top amp board, which does have several big caps in a corner. Also, from there, a couple of thick wires lead to the preamp board underneath. I don't know if they're supposed to carry power with them, but I have a hunch they do.
I hope to find someone who's willing to do some measurements on their box for me, lacking a service manual. I'll create a new thread by then after some more digging.