Good point, I am focusing on ‘custom’ SUT’s that are designed specifically to mate my choice of the cart’s.
@lalitk OK- keep in mind that if you have an SUT designed for a particular cartridge, if you change out the cartridge, you may have to change the SUT too or figure out the right loading. This will also be true to a lessor extent if you change the tonearm cable, since the capacitance of the cable is part of the loading of the transformer.
Its these little details which are really the downfall of analog, not the performance or the sound!
So if you get a custom transformer built, ask the manufacturer for tips for loading it for best performance. If he tells you there's no such information or you don't need it, consider looking elsewhere.