Stereo to Mono
Great question. You would still design the enclosure the same way. I am not familiar with Bass box pro, but it usually gives you the Q of the box. Assuming a flat response, you would want t a Q of around 0.7
Why would you want the sub-woofer to go up to 250hz?
This is what a 250hz tone sounds like .... https://szynalski.com/tone#250,v0.75
250hz is actually a mid-range frequency at the bottom of the male voice range. The only time I've seen such a high crossover point in a "sub-woofer" was in a set of those icky plastic 2.1 Logitec speakers, and that was because the satellites couldn't go any lower without severe distortion.
Don't forget a sub-woofer and a woofer are two completely different animals, both in structure and in use.
Sub-woofers usually stop at about 80 to 100 hz.
This is what 80 hz sounds like .... https://szynalski.com/tone#80,v0.75 (You may need headphones to hear it, depending on your computer speakers.)
Generally in a 3 way system you want your woofer to get to about 250-400 hz (depending on make and model), the squacker (mid) should run from the top of the woofer up to about 2800-3300 hz where the tweeter takes over.
In a 2 way system your mid-bass would go up to about 2400-3000 hz where the tweeter takes over.