Okay, I finally sorted out my mistaken placement of the input resistors in the Foreplay III. Naively following only my intuition I had installed the resistors symmetrically, which looked good but padded the left channel of input 1 and the right of input 3. Since I kept changing inputs and wires, thinking it was the ST-70, it was a moving target!
So, back to the ST-70, yes, I have about a 2:1 voltage divider padding the inputs now and I'm pretty happy with that. I'm looking at modifying the inputs per the Van Alstine page mentioned above, so I'm not sure how this divider will work with that, but in the mean time I would appreciate a recommendation to optimize or simplify this voltage divider with just two resistors on each channel.
I am also curious, having bypassed the preamp in the troubleshooting, why my phono preamp (Hagerman Ripper) when connected directly to the amp is, or can be, very loud compared to the Foreplay. It seems that the Ripper is designed to do double duty as phono + pre-amp. It appears about the lower half of the volume steps are attenuating the inputs, and above that they're amplifying. Is this happening with my CD input, and is this normal?