I'm thinking about replacing an OT so and was looking at the order page on a couple of transformer sites the other day. As I read through the descriptions they are typically identified as replacements for a given amp or circuit. Alongside those are the special order winds and components offered as custom orders, let's ignore those for the moment. My question, what characteristically differs between "types" of OT? Specifically, what would you get from putting an OT identified as a blackface replacement into a tweed or vice versa. I understand that each is designed around the parameters and spec for their given circuits, but assuming that the voltages, draws, recoveries, etc. were both within an acceptable plus/minus range, what would be the difference between a tweed transformer and a blackface one? In many cases all I can see is difference in the secondary, e.g. that a tweed wants a 4 ohm speaker load and not a 4, 8, or 16 option. Are there other, basic, differences not listed in the spec; type of wire, number of or way of winding - something you amp builders already know that I can't tell from the description? In my particular case, the correvt replacement is a Mgnetic Compoenets 40-18001. If I wanted to get to get a more tweedy tone, could I do that with a different OT, or is it just a minor part of the equation?