I will summarise hoping to answer the questions.
We have a couple of very expensive valve guitar amps. A large part of their sound characteristic comes from their output stages, both at normal levels and overdriven.
Under normal listening conditions indoors, we play very quietly, thus missing out on the true performance.
I want to design a dummy load box to allow us to drive the output stages, while still using the same cabinet/speakers, but at comfortable listening levels.
Aware of the limitations of this setup, we have run tests with hard wired resistors etc and are very satisfied with the result. Additionally, we have decided that 5W is the absolute maximum we ever want to send out to the cab/speakers.
The problems:
I cannot think of a way to send only some of the signal to the speakers, and with a volume control independent of the guitar amp, other than by tapping it off of the load resistor and then re-amplifying it with another amp. I have thought of using bulky rheostats but I can't come up with a decent, totally passive circuit.
Since we only need 5W to fill a normal room and still annoy the neighbours, I only need a small little amp inside the box, like the LM1875 for example, it will be plenty.
I use a small, step-down transformer, with a windings ratio according to the selected load, 4R, 8R or 16R (that is why there are three primaries on the schematic), so as to keep the tapped signal level to standard +4dBu (~ 1.25V RMS).
As my first question I am just not sure I am doing the tapping properly, it simulates OK, but maybe there is a better way?