Is 8.7hz too low for tonearm/cart resonance?

I'm no expert, but everything I've read over the years (and I've read a lot) indicates that the preferred range for tonearm/cartridge resonance is from 8 Hz to 11 Hz. So according to that, you should be okay. I'm not sure if your floor/footfall situation presents any problems.

-Bob

8-11hz is optimal, you're fine.

If you know the effective mass of your tonearm, you can go to cartridgedb-dot-com and double check everything with the cartridge resonance evaluator there.