This is how I added a sub ^^^^^^. Pulled the stove circuit and used that spot to power the sub. On a different note, the OP may need 4 spots to power a 100A circuit though. I did on my FP Stab-lok panel.
OMG if you have federal pacific panel it may be in your best interest to replace it.
Do a search on federal pacific breakers not tripping. Zinsco are just as bad.I know it's probably been there 40 or 50 years and worked perfect.
But when the current rises over the breakers trip range and it does not trip... melting insulation, over heating of the conductor going thru bored holes in wood, will not end up good.
I change them out frequently with Siemens or Cutler hammer panel boxes.
