I've looked at the circuit design of the SMK-H8K mics and it appears to me that 30 V of power supply voltage will work fine for these mics. The harmonic distortion in the amplifiers contained in the mic bodies would be slightly increased, but I doubt that you could notice the difference by listening. It might be possible to see a difference on a spectrum analyzer that's looking at the mic signal. You'd feed an acoustic tone into the test mic and look at signal levels at integer multiples of the tone's frequency, first at 48 V bias, then at 30 V bias. There will be some measureable difference, but again, I seriously doubt that you would be able to reliably discern the difference by listening to the signal recorded from the mic under test.