PB was posting while I was writing, but here are the underlying calculations.

Speaker amps like Kaiju are designed to transfer maximum power into a speaker load. Kaiju can be wired to drive 8 watts into 2, 4, 8, or 16 ohm loads. Using higher impedance phones is not a problem, but the available power in watts decreases as the load impedance increases. For these higher impedances, maximum voltage is a better measure to use. When wired for 16 ohms, Kaiju can put out 11 volts RMS.

Headphone amps like Mainline are usually optimized for higher load impedances. Mainline can be switched to drive 600mW into 32 or 64 ohms. At higher impedances, it can put out 6.2 volts.

So Kaiju can put out 11/6.2 = 1.8 times as many volts into headphones with impedances over 64 ohms. Power is proportional to the square of voltage, so that's 3 times the power of Mainline.

For impedances around 32 ohms, Mainline should use the lower-impedance setting, which reduces the available voltage to 4.4 volt (half the power) so Kaiju is 6 times the power of Mainline.