9 gets you to 4 ohms. Put three woofers in parallel, three times, you get three triples each 1.33 ohms. Put those stacks of three in series with each other and you're back to 4 ohms. That's why people call it the "magic square", same number in series as in parallel.
4 woofers: (two squared) two in series, two sets in parallel (or vice versa): 4 ohms
9 woofers: (three squared) three in series, three sets in parallel (or vice versa): 4 ohms
16 woofers: (four squared)
25 woofers: (five squared)
If you do eight 4 ohm speakers, your options are 2 ohms or 8 ohms, or (two 4 ohm loads... which bridged amplifier wise is same as one 8 ohm load).
Right now you have 4 ohms. If you add two more on your existing boxes you'll have 8 ohms (or 2, which you can't run).
Last edited by Chris Popovich; 07-17-2019 at 12:27 PM.