This isn't going to help much... But... Speaker distortion can be ignored. Most distorted amps, classic or otherwise, do not actually drive the speaker hard enough to cause speaker related distortions. There are a few. But it's rare enough that adding this parameter to your criteria only complicates your problem. So that leaves one more parameter. Air movement. Which is to say, how the human ear percieves frequency balance and compression at different volumes. This is certainly something that can be simulated with frequency response, BUT... It's frightfully hard to emulate all the nuances with a device as crude as a power attenuator. That's why most only offer a simple bright switch. The HotPlate has a lamp circuit that offers some compression. But it's my understanding that most players only use it until the novelty wears off. So we're left with only EQ> The Weber MASS unit has adjustments for high, low and mid. But IMHE players become confused by this. It just seems to get more lost the more they mess with it. Looking at the schem for that unit I can see why. There isn't much isolation possible for the band specifics and this just makes it hard to dial in. The average musician would be much happier with one knob that makes things sound acceptible. You can search for the magic formula if you like. Others are doing it too. It's commendable that you want to make a higher performance product. I just don;t think there is an economical way to do it in this case. The Ho, or Ultimate attenuator uses an impedance that is somewhat nominal to speaker impedance over a large bandwidth and then re amplifies with a solid state circuit. This allows much better EQ control. Food for thought. The Ho/Ultimate attenuator is considered by many to be the best sounding unit on the market. And it should be for the price. But consider that the human ear s perceptions change with sound pressures. So any EQ adjustment would need to be controled logarithmically simultaneaous with the amount of attenuation. That's my two cents. Good luck in general. And good luck with the translation of all the vocabulary I've included.
P.S. I failed to mention that you neglected one more parameter that affect attenuation performance. That is, speakers have a complex impedance. Resistors do not. An amplifier playing into a complex impedance performs and "feels" different from an amp playing into a purely resistive load. Since a reactive load that matches every speakers impedance curve is impossible, it is therefor impossible to create an attenuator that truely sounds accurate at high levels of attenuation for every amp. Not to mention the much greater expense of including reactive circuitry in such a design. There will always be a compromise.