So which is it, 100hz XO or 400hz XO? The ripple is only one issue, it's not a woofer it's a subwoofer. The impedance has doubled due to voicecoil inductance within the passband, you're going to have Le(x) modulation out the wazoo.
Also, we have the 1 measurement. Breakup and resonances behave differently on and off axis. Use it if you want, but neither I nor seemingly anyone else other than you think it's a good choice to 400hz. This is a timely discussion as like you, I would love a driver that could do 20-400 in 4 cubes and even looked at the Ultimax, but the inductance is the big dealbreaker for me moreso than the ripple. That ripple's going to likely move around a bit in use, creating a haze that your XO won't suppress.
Also, we have the 1 measurement. Breakup and resonances behave differently on and off axis. Use it if you want, but neither I nor seemingly anyone else other than you think it's a good choice to 400hz. This is a timely discussion as like you, I would love a driver that could do 20-400 in 4 cubes and even looked at the Ultimax, but the inductance is the big dealbreaker for me moreso than the ripple. That ripple's going to likely move around a bit in use, creating a haze that your XO won't suppress.