Does a Subwoofer Make Spiking Redundant?

Not the subwoofer, the main speakers. Or rather, the main speakers were spiked before I got the subwoofer. If it's true that they're two different things then that's the answer to the question. 

Never use spikes. Use springs like Nobsound, or even better Townshend Pods or Bars. Or the very best, Podiums. I’ve compared and Townshend are by far the best. Spikes, the worst.

Then mids will still cause vibration. The type of food the speakers are on matter too. Carpet over concrete or wood etc. 

What you are trying to do is fix a small mass to a much larger one to drain away excess vibrational energy, to dampen resonance. It is the exact same principle you use when you fix a cartridge to a tonearm. Ideally you would bolt the speakers to the floor and I have seen people do this. The problem is the mass of your house is not solid. It vibrates also. Here a concrete floor is also a big plus but an option many of us do not have. After bolts spikes are the next best thing. Both subwoofers and main speakers should be fixed although it is more important for subwoofers. Putting speakers on isolation springs is an odd thing to do for two reasons. This allows the speaker to vibrate itself silly and the floor is going to vibrate just the same from the acoustic energy released by the speakers. So, if you want the ultimate, bolt your speakers to a concrete floor.

Unless your floor is concrete, I would think isolating your subwoofer would be better. With standard wood frame construction, you don’t want the sub to couple to the floor which then acts like a radiator vibrating at its resonance frequency, mucking up sound quality. When I isolated my subwoofer from the floor, sound quality went way up. I noticed that it didn’t interact with the room as much - much less vibration coming thru the floor. If your goal was maximum loudness and impact, maybe you want that effect, but you don’t where sound quality is the objective.