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After putting this transformer in (replacing the one in the 1959slp reissue), the harmonics just jump out effortlessly now, whereas before it seemed like there was something blocking them and the amp just felt cold (even when biased hot!). There's a lot more detail and clarity to the sound, and it seems to have removed some harsh frequencies too.
Before I put the OT in, the amp sounded like it was cloning a Marshall sound, like it was attempting to get that sound but was kindof missing it. Now it sounds like it IS a vintage Marshall, and with some NOS tubes I'd put money on people not being able to tell from hearing it that it's not a vintage 1959.
It's by far the best upgrade I've done in my amp. I wouldn't even think about doing a build with a different brand of OT, this is where the tone is at.