You shold be using 70 volt product in your 70 volt system. I don't know why that even needs to be said. Did you think there aren't any 70 volt subs?

We have a GREAT system in a wine bar/restaurant with some JBL Control speakers and a couple of Control subwoofers. Sorry; I don't know the model numbers as I only worked on, not sourced, the system. But the subs are a bit larger than and the same shape as the main speakers. Crossovers are built in. Like, voila, it's done!

We also have some others in a large bar -- two inwall subs, six speakers -- in a ceiling above a bar area about 40 x 60. It totally kicks, and the input to the sub is just another 70 volt speaker wire.

Meanwhile, if you're using a sub designed for consumer use, you'll likely have hum problems, and you'll be dealing with signal levels much higher than those in a consumer system. In a home system, if you had a 100 watt amp and you turned it up so it put out 100 watts (into 8 ohms), you'd have 28.28 volts. If you've got a 100 watt 70 volt amp, 100 watts will be seventy volts. Just be sure your sub is ready for that voltage level on its input.