Kinda Long

I have a couple of 604-16X (ceramic-mantaray, 16 ohm & tangerine plug) with symbiotic (sp?) HF diaphrams. Bill at GPA can replace the Symbiotic with aluminum, recone the LF & make all 8 ohm. I would then have a 604-8K. This is great except, I think I remember reading that early ceramic 604's had problems with quality and pole piece / motor configuration. Does anyone know at what time (year of manufacture or s/n date code) Altec finally worked out the ceramic motor / quality problems? I would hate to invest the $$$ to convert just to end up with something significantly less that a real 8K. TIA! Manzanita