55-60hz is about as low as you should ever realistically need to LP your subs. 35hz is way too low. Dont go from expecting your subs to play midbass, to expecting your midbass drivers to play sub freqs.
But cabin gain in a vehicle is rarely a smooth rolling increase of 12dB/oct like we would like to think it is. So sometimes low Q alignments or even .707 alignments get peaks and valleys themselves. I know that the three transfer functions I have measured (99 Acura Integra hatch, 93 Nissan 240sx hatch, 93 Mitsu Mirage 4 door) have given very non linear curves. The hatchbacks had three distinct peaks starting in the low 20s then in the low 40s and then in the mid 60s. The max flat sealed alignment would then usually give me peaks in the 40hz range because the gain was about the same there as it was in the 20s but the sub had already rolled off ~10dB at that frequency when measured anechoically. The 40hz range was almost at reference when measured anechoically (I don't think it's a word but I've used it twice so **** it haha) so it left a high gain there and a shallow roll off below it. It was like that in both the 240 and the Integra. The Mirage was weird and I don't remember it very much anymore besides real high gain at 45hz.
I disagree for some cases but for the most part I agree. I am with you on not having the mids play sub frequencies but sometimes a low LP filter can be very useful in a vehicle. It can work as a pseudo-Linkwitz Transform by bringing down the levels of the upper bass that a smaller sealed box will perform very well at and with extra power (if you have the headroom on tap and mechanical limits down low), you can reinforce the bottom octaves and flatten the response.
why have mids playing sub freqs?
