The brittle sound could have been a bias or power tube mismatch issue or even a microphonic preamp tube: no shock isolation on MTI's. They also used 12AX7's in place of 12DW7's, and a 12DW7 has a much higher current output, so it will sound beefier and provide more current (low end) to drive the power section. If you had a bunch of 90's Chinese 12AX7's in there, that could also be it. They're ALL icepick-in-the-ear toned.
Once the preamps are 12DW7, there is no appreciable tonal difference between my MTI and any of my other SVT's. SVT's are like Chevy big blocks: they'll take tons of abuse and run nearly forever, but once you've experienced one dialed in properly and like it, you're ruined. GE 6550A's put out more power than the old Tungsols, but aside from having a very slight lift and more punch in the upper mids from better peak power, aren't going to make an SVT sound substantially different. Magnavox, by far the largest producer of the SVT, shipped them with both Tungsol and GE output tubes bearing their logo. The only output tubes I've ever had to have a pronounced tonal effect in the SVT are 90's Chinese KT88's (and those same KT88's rebranded as KT100's), which tend to be darker and more murky tonally, but even that is just a mild coloration.
Baz, serial numbers with Ampeg are generally meaningless. Got pics?