i made an assumption about the center tap (but i am in the Ben Franklin stage when it comes to electronics) and cannot read a schematic. thanks. i know the jack is a switching jack of some sort because the second jack will not work unless the first jack is used. maybe a simple on/off type switch.
Fender used shorting jacks to protect the amp from no-load situations. Until you insert a plug into the primary output, the amp is dumping its output directly to ground. They're technically not a switching jack like in the original Ampeg SVT's extension jack, which selects between separate OT taps.
the 12's are rated at 75 watts each, the JBL is rated at 200 watts @ 8 ohms. both are sensitive speakers at or above 97 db @ 1 watt.
Keep in mind that 97 dB is at 1 kHz; at bass frequencies you'll be lucky to have 80 or 85 dB.
so bottom line = 8 ohm cab then no other cab and 4 ohm cab then one other 4 ohm cab is okay?
Yes, HOWEVER you
could go lower in impedance-eg more speakers, but you will reduce your output power and wear the output tubes faster. Since tubes are relatively low current devices, lower than rated loads aren't the danger to them that they are to transistors. In most cases, you'll lose any volume gained by the extra speakers to the decreased headroom of a lower load, but it depends on the speakers, the OT, the tube quality, their current and the amount of gain you use. IOW, you could try it and see without it really hurting anything. If you see your output tubes' plates start turning colors from overdisappation, then you'll know not to do it, but if your amp is biased fairly conservatively, it might be just fine. I'd wager it won't be a good idea, though. Where your speakers are paralleled, they will have a greater voltage drop than a series or series/parallel combination.