The sub box is 2.0 cubic ft per sub tune at 34hz with Sundown sa 12. I've heard that with tunning below 35hz your lpf should be around 80hz or below! Any truth to that? And shouldn't the lpf slope play a part say if 80 hz was best is that with a 12db slope or 24db or does that even mater?

That's a bit of a complicated question, but the short answer is no, the tuning of your sub enclosure shouldn't effect where you cross over to the sub at, generally speaking.  As with everything, there will always be exceptions. 

Edited September 9, 201510 yr by Triticum Agricolam

I have heard tuned from 30 to 35 never go past 65-80hz lpf and if tuned from 40 to 45hz set lpf from 100hz to 120hz on 12's... I have just always thought tunning would affect where you set your ssf not lpf.

I have heard tuned from 30 to 35 never go past 65-80hz lpf and if tuned from 40 to 45hz set lpf from 100hz to 120hz on 12's... I have just always thought tunning would affect where you set your ssf not lpf.

 Go with your last sentence an call it a day!!!

I have heard tuned from 30 to 35 never go past 65-80hz lpf and if tuned from 40 to 45hz set lpf from 100hz to 120hz on 12's... I have just always thought tunning would affect where you set your ssf not lpf.

 

Your thoughts are correct, tuning should affect where you set the SSF.  LPF will be more of a system and user preferenced setting.  Some drivers in some alignments will do well playing up higher, others won't.  Sometimes if the front stage is weak a person will need a little higher LPF setting to get the midbass response they are looking for.  It's really pretty situation dependent and basically a person should just set it to what sounds good to them.

SSF is a high pass filter and like said ^^ is set according to your enclosure resonance.

 

For your lpf question, the answer is an unequivocal "NO"...