The Helix does 4 x 35W at 4 Ohm for the main speakers and 160W at 2 Ohm for the sub output.
The Helix sub box, contains two 6" subs, dual 2ohm voice coils (a few ways they could have wired this, but the end result would present a 2ohm load to the Helix).
I don't listen at high volumes but it's a safe assumption you would be power limited. The output from the stock radio does not clip, even at max volume. But the stock radio does have EQ tables in the car specific dataset so it accounts for the limits of factory speakers. Quite possibly the dip you noted at 60Hz is by design. This link is way into the weeds, but talks about factory dataset EQ...
https://mqb-blog.com/en/2022/02/21/sound-dataset/
I didn't check the Helix output so don't know if it clipped at high/max volume. But it's worth keeping in mind the Helix would not have completely "undone" whatever compensation was made in the factory EQ tables of the MIB since the stock speaker (relative lack of) ability would still be a limiting factor.
The AudioControl D series has built in RTA and it will flatten all input, at which point you can then EQ as you see fit. The L series does not have this. You will also lose time alignment with the L series.