Right.
I've done my sums properly (ish) now. All based on linear assumptions about impedance, of course.
If I have a single 8 ohm speaker producing (or perhaps consuming would be a better expression) of power, it's going to draw 4.33 amps at 34.64 volts.
If I have two 8 ohm speakers in series, the impedance goes up to 16 ohms. With the same peak voltage of 34.64 volts (assuming that's all the amplifier can muster), the speakers will draw 2.165 amps (half of 4.33 amps for twice the impedance). And each speaker will produce. sorry, consume, 2.165 x 2.165 x 8 = 37.5 watts, giving a totla consumption of 75 watts, or roughly half the power of just one speaker, not two. To get 150 watts total (75 watts per speaker) you need 3.06 amps, meaning the voltage drop across each speaker would be nearly 24.5 volts for a total across the amp of 49 volts, which I'm not sure many amps could muster.
If you have 2 x 8 ohm speakers in parallel with no ballast resistor, and apply the same 34.64 volts, you get 150 Watts per speaker, a total of 300 watts. BUT your current demand goes up to a whopping 8.66 amps. Again, I'm not sure most amplifiers would handle this. To get 150 watts total (75 watts per speaker) you need 3.06 amps per speaker again, giving a total of 6.12 amps and only 24.48 volts to drive the speakers to that level. so you have to drop (34.64-24.48) = 10.16 volts across a ballast resistor. 10.16 volts and 6.12 amps means a 1.6 (ish) ohm resistor.
Answer? Trim the center channels down a bit, or if you're a bit Mutton Jeff like me, leave 'em alone and enjoy the clearer voices, remembering the decibel curve is logarythmic not linear.
Glad I sorted myself out on that one.