I said in my last post that I don't disagree about impedance interactions. However they are not what gives the character to a tube amp. If it were only amplitude changes, then you could simply use an equalizer to make any old solid state amplifier sound just like a tube amplifier. Not likely.

Of course unloaded circuit networks have existed forever, but it has become the de facto interface model with the advent of solid state electronics which can achieve low output impedances with ease, this especially being the case with IC Op Amps.

Again, have you tested your equipment with lab equipment? I think that would sort out the problems you are having more than any amount of conjecture.

In any event, if you are concerned about output impedance of your preamp, why don't you simply replace the buildout resistor with a different value, all the way down to zero ohms if that's what you like? Similarly, the input impedance of gear is generally established by a resistor at the input of the input buffer stage - why not just replace it with a value you deem appropriate?

By the way, active crossover systems don't have to use "impedance flattening circuits" or any other compensation mechanism. In this type of setup, the diver is connected directly to the output of the power amplifier, so impedance swings are limited solely to that of the driver, and these swings tend to be very mild compared to a speaker system with complex passive crossovers. The crossover is at line level, before the power amplifier, so there is no possibility of interaction with the speaker's impedance magnitude (my system has been actively tri-amped for decades).