How closely does this amp come to actually providing 70 volts at full power into a load? We're used to thinking in terms of 8 ohm loads or 70 volt topologies, but the two modes are just different ways of understanding how much power the amp can present to a load.

Start with one of the formulas for power, say voltage times voltage divided by load gives you watts. This converts to "square root of watts times load = volts."

The specs for this amp say:

375 watts x 2 at 8 ohms (they mean two channels). 375 watts into 8 ohms is 54.7 volts.

650 watts into 4 ohms is 50.9 volts.

1050 watts into 2 ohms is 45.8 volts.

This tells us that the amp can output A LOT of voltage, in fact impressively close to 70 volts directly from the output stage! It also tells us that as the output power increases, the output voltage decreases. This is probably to keep all components in their SOA (Safe Operating Area, the combination of current and voltage level [that is, heat] below those that kill the output transistors).

Some arithmetic will show you that an amp outputting power at 70 volts into 8 ohms is putting out 612.5 watts. It's occasionally helpful to know about this.

In your case, the answer is that the amp will indeed drive four 70 volt speakers, but it'll drive them at maybe 55 volts. Maybe 60, since we don't have SOA information for that exact load. The volume of the speakers will be slightly lower than a 70 volt amp would produce. I believe we're talking one or two dB less, though. Some more math would clarify that.