I was only noting that technically speaking, the preamp includes all of the signal shaping functions of the volume, bass, treble, etc. controls. Preamps do have built in amp stages which boosts the signal before being cut again by the bass/treble controls. Contrary to popular belief, the tone controls do not boosts either bass or treble when you slide them up. Because this is a passive system, those are selective signal cutting controls. When slid all the way to the top, they allow all of the signal to pass (all of the bass and treble). Instead, in their mid position, they attenuate the signal and that signal is what the output amps amplify to the speakers. In other words, in their mid position, each of the tone controls have attentuated the signal strength roughly 1/2 (not linearly). So actually, they boost nothing. Where they are in their relative position indicates how much of the signal is cut and when bass/treble are both in the mid position, the sound should hopefully sound balanced again. So since the function of the preamp is to cut the signal and not boost it, it therefore requires that a signal be amplified before being cut. The boosting stages inside the preamplifier itself are commonly called line-amps, buffer amps, or yes, pre-amps.
What does this mean for you? Well, the M70 has several preamplification stages and then finally, a line amp which boosts the signal to line level. Both the tone control module and the line-outputs get their signal after all the preamplification has already taken place. Since you are getting stereo at the line outs, then it appears no preamplification stages before the actual PREAMP (tone control board) are suspect. So it would seem that your issue is most likely either a failed volume slider, or catastrophic failure of the treble slider, or interconnect system, or a failed output amplifier system {and I say system(s) because there are appurtenant components such as resistors and capacitors in the signal path too}. If you really want to try and short the left/right side to see if the issue is with the amplifier module, pins 7/12 should do it for you. One is the normal signal input, the other is the phase inverted signal input. Since this is a BTL system, each module has 2 amps and one amp boosts the normal signal and the other amp boosts the inverted signal.
