Distortion at 120 Hz doesn't matter, because we usually cut our subs at 80 Hz.
If you look them closer, you'll see that at the same level (for example at 95 dB) Sig servo actually has much less THD than the older servo. THD rises only at very high levels below 20 Hz, which older servo can not even produce.
As said before, at the time sig servo starts to compress, older servo has bailed out long time ago. Look at the 10-12 dB output difference above 30 Hz and 5 dB difference below! 6 dB means doubling up the number of subs.
I know reading those measurements can be very hard, but actually the newer servo is much better sub than the older one. At least when looking at those measurements.