This is my first time posting here on this forum, although I have been a Circle member for a while. I have to admit that I don't know a lot about passive preamps. However, many years ago I did own the original Line Drive from Mod Squad. From my reading so far here and from other mentions elsewhere, I see that that these newer designs are an entirely different animal all together.
My system is currently passively triamped. Well, that is to say my main towers (VMPS RM-V60s) are passively biamped, and the separate bass towers use mono amps with built in crossovers. Still, my tube preamp is seeing the load of three amplifiers in parallel. The total impedance is very low, a lot lower than tube preamps want to see. I have helped out this situation by taking the sub bass amps out of that parallel circuit by running them directly from the output of my higher frequency VTL monos. To do this I made a bridging circuit, or a voltage divider IC dropping the VTLs output back to line level. Now my preamp only sees two amplifiers in parallel for each channel.
This arrangement may change in the not too distant future by going active instead of passive, but for now this is my world, and I like my world for the most part. My preamp was an Audio Research, but I am now using the built-in preamp of my Monarchy NM24 tube DAC. I think this is an excellent DAC and the preamp isn't bad either. But, I have read that its volume control takes away from an otherwise excellent line stage, and by using something like what you offer here will really increase it to its full potential, and the differences are not subtle.
So, that brings me to a few questions. What about this impedance issue of driving several amps in Parallel with an 18' interconnect? What is the lowest amplifier input impedance your controls can see without damage to the music? Would step up transformers be helpful in bringing up that total impedance values?