My reply contains all technical and logical reasoning. No pun Intended here.
The “Fs” stands for “free air resonance”. When the sub is loaded into the enclosure than the Fb becomes the “Fs” in vented alignment and “Fc” in sealed alignment.
It is helpful to look up Q and understand what that means, its pretty simple, the Q rises as the box gets smaller and 'humps up' the response you get. 0.707 is the flattest response. Also the Qts of the sub determines how it responds around Fs, how it will respond to changes in the box as well. This is how some speakers are made for boxes and some are not, or some small and some need large boxes. If you figure out how those few things work you will understand what the model is telling you a lot better.
The spl champ is right if it was tuned for db drag or for SPL conditions. OP wanted the Sql type enclosure for this sub. If you tune it to 42Hz, it will peak around 49-50Hz. That’s a painful threshold for listening; it would be great for only banging Indian pop and vibrating the hell out of car interior. And that brings the need of subsonic at higher frequency which would be around 35Hz. How can one live with a subsonic this high whereas the music instruments on stage get dirty low as 5Hz.i.e Pipe Organ.
It is a tradeoff, if you want SQ and don't care about loudness than tuning the pioneer in low 30’s is the way to go.
Tuning below Fs is absolutely fine and everyone out there in international world do that all the time. What if I say the Fi BL’s have higher Fs in the 35Hz to 39Hz region and manufacturer recommends tuning them lower than their Fs. Even Cerwin Vega subs have recommendation of tuning lower than their Fs by the manufacturer .i.e. Vmax 104 and also the specs on a Cerwin Vega Stroker 12 has an Fs in the high 30s/allmost 40 but their recommended enclosure is tuned to 34hz and down 3db at 26hz.
http://www.cerwinvegamobile.com/manu...anual-2009.pdf
So it cannot be generalized that the tuning below the Fs is not recommended. Group delay is most audible in 40s Hz region whereas in 20s Hz region its not audible and neither to be worried about.
If you see the impedance plot, the impedance is at highest at the Fs while the 2nd impedance peak is followed in vented enclosure due to the rise in Q. So for a recipe which I believe on and what physics has taught me, take the enclosure size above VAS and tune below Fs, this way the PR of the sub flattens out at the expensive of SPL. Tame the low end by EQ as it dovetails the cabin gain. Splfanatic tuned his GTO’s to 29Hz which played flat till 55Hz as he told me, also we all know the famous loudest santro with 4 309’s S4, which got low till 20Hz and the mighty enclosure was above the VAS.