Okey
In the early days of Marshall amplification they often used KT66 tubes together with the Drake 784-103 OT which had a primary impedance of 8kOhms (for a pair of KT66) - JTM45 originally was equipped with this OT, therefore default configuration of a JTM45 must be KT66 instead of EL34
If we compare this to a common Z of 3.2K to 3.5k on an EL34 OT, this would result in 0.43 as your tranformer matching factor.
That brings me to another topic, Cliff might want to chime in. There are some parameter most of us wouldn't touch or even thought about, I remember Cliff saying once he would hide them because the fact nobody touch them (I begged to let them stay in) - Power Amp Bias Excursion, Recovery Time and Excursion Time. Those will change when another amp is selected but they stay if you choose another tube (and regarding to Cliff also the OT and the PI). If the coupling caps and grid leak resistors remain the same, okey - these parameter wouldn't have a change. I would say, this should be a further topic for the Cliff Notes section, but here is a little preview à la Paco :mrgreen
To explain them quickly..... a high overdriven tube goes into grid current which charges up the coupling capacitors now adding extra voltage to the grid - if harder driven, the bias point get lower and the amount of blocking distortion rises. The result is known as "Bias Excursion". Now there were two other parameter "Excursion Time" (how long the excursion concours) and "Recovery Time" (time constant of the coupling cap and the grid leak resistor). Increasing Bias Excursion and recovery time will make your sound weak, fizzier and fart-out on harder attacks (palm mutes riffing and higher MV settings). It also affects the harmonics in your overdriven signal in a special way ....lower settings were used as default on various amp models and lowest on soild state amps sims (there is no such thing - that's why the JC120 has no amount of "bias excursion" at all). You can boost this effect by lowering the bias point (Pwr Amp Bias) which also makes the sound a bit more explosive on harder riffage (especially on classic marshall amps IMO)
The coupling cap capacity does not just define the amount of possible bias excursion on the following driven tube, it also affects the amount of low frequencies. More capacity = lower frequency pass and more possible bias excursion. (As an example - Hifi-tube-amps would have maximum possible bias excursion - but since they were not driven that much, you'll never hear any such things. But if you want to show the ugly face of your tube fanatic hifi-freak - turn up his stereo and make it worse....hahahaha :lol )For an even detailed simulation you can also adjust the "Pwr Amp LowCut" parameter up to personal taste.....
There many more things - but I only can assume how the simulation is working....
Cheers
Paco