Hey Wayne/Martin, Lots of info in your response Martin.
The early Marshall RS Spares OT was a cheap Hi-Fi OT made for RS by various companies. It was only rated at 30W though and the JTM45 will push a little more than that. This gives a great saturated sound to the amp when it's cranked. The laminations in this OT were M6 as opposed to the junk steel used later on.
The next generation OT's from Drake were rated at the same wattage so the same saturation occurs but they used low grade steel and were interleaved completely different. This effects the tone of the OT and the amp. There was definitely a lean towards less need for response at frequency extremes. This OT was also made with a higher reflected primary Z and this alters the amp sound as well.
Hi-Fi amps and OT's are, in my opinion, designed around a wider frequency response than guitar amps and proper loading for optimum power and fidelity.
I haven't considered how the newer breed of amps are made as far as a wider frequency range in their OT's. I would imagine that a better speaker setup would be required to take advantage of this as opposed to the standard 2x12 and 4x12 setups.
I'm sure I'm just repeating the previous response here but I'd thought I'd answer anyway.
Yup, any fool can build an amp. ;)
Brian