KT66s will work sonically as 6L6 subs, you might like them and you might not. I would be hesitant to use them in your amp, however, since they draw a lot more filament current than 6L6s. I don't remember if the '90s Model T has an EL34/6L6 or 6550/6L6 switch - if it does, then you'd be OK. Otherwise I wouldn't do it.