It's not pot tapers, if it was pot tapers you could adjust the knobs until you got to the same value on the different tapers. The taper of a pot is just the shape of the ramp up between values. A 10 MOhm pot goes from 0-10 MOhms no matter the taper, the taper just decides how quickly it ramps from 0 to 10 as you twist it, so it has a big effect on what the half way point value is compared to other tapers. But all pots that have the same rating start and end at the same palce, and have all the same values available along their travel, just at different places.I think the SLO is incredible with a drive in front. You can really create the ultimate high monster with that thing. I always think about the fact that it is the basis for both the 5150 and the Dual Rectifier (and I guess the Archon too), even though all three amps sound very different functionally due to very different pot tapers. So I think of it as in that family anyway, where you can really hone your own beast with a drive. I wouldn't know what to do without a drive on that thing. I've tried!
There are a few circuit tweaks between them, but the design of the power amps and negative feedback is a big source of the differences.