Just wondered what peoples options on the ideal hpf for these ..

Im guessing something like 40hz 18dB 

Is there a formula to calculate this? Also wondering about the pro's and cons of different slopes ie 12/18/24 dB per octave..  would 12dB slope provide enough protection? 

Any thoughts on this will be much appreciated..

You can get enough protection from a shallower slope like 12dB, but you usually have to set the corner frequency higher to get it, which can eat into the response within the intended passband of the subwoofer. A steeper filter can be pushed to a lower frequency for a given degree of protection, potentially avoiding that problem.

However, steeper filters have more phase shift and thus group delay, so may be perceived as slower or less tight sounding.

One happy medium is to use the lower order filter with a little boost from a PEQ just at the box tuning frequency to restore the lost level there - that gets you all the in-band response with a little less group delay than the steeper filter.

The differences aren't huge to be fair, but they're interesting to be aware of anyway.

Here's a 186-2 in 172 litres tuned to 36Hz (approx what you'll have, depending on how much space is used up by handles, bracing etc) with a few filter options.

HTH,

David.

Edit to add: the yellow trace for the 12dB filter on its own has it's input signal turned up a bit to allow for the fact that the HPF has reduced excursion in-band as well as below box tuning frequency.