Does a Subwoofer Make Spiking Redundant?

"Does a subwoofer do the thing spiking speakers is supposed to do?"
Are you asking if it is worthwhile to spike subwoofers? If so, according to many who use them, it is a good thing. I think you're just asking the question wrong. Spiking and subwoofers are obviously two different things. 

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.