No worries .
I only play around with sealed woofers which makes things a bit simpler, there are a few different methods and in my experience they all mostly line up.
1) Measure in enclosure T/S parameters with Dayton DATS. Specifically with QTC and Fc you can predict the 2nd order high pass behavior.
2) Measure acoustic response with mic very near to woofer dustcap.
3) Rely on measurements of others. For example Linkwitz has done outdoor / ground plane measurements of the LXmini indicating LR2 @ 60 Hz roll off. Dave Reite reproduced these when design the LXmini+ add on subs.
Once you understand the roll off you can also modify the roll off behavior using a Linkwitz transform and/or additional electrical high pass to obtain your desired roll off. For example I could use a Linkwitz transform to change the LR2 (Q = 0.5) @ 60 Hz roll off to BW2 (Q = 0.707) @ 60 Hz and then apply a BW2 @ 60 Hz electrical HPF and this would result in LR4 @ 60 Hz roll off.
The other variable to consider is that delay as a result of path length differences between the woofer and subwoofer will change phase response and impact how the crossover sums. Linkwitz discusses this here -> https://www.linkwitzlab.com/frontiers_5.htm. In general this analysis treats the subwoofer as just another driver that requires proper integration with the woofer. I also consider the high pass behavior of the subwoofer in the crossover design by applying a HPF equivalent to subwoofer rolloff to the woofer for better phase matching.
To me it is not so much that the crossover negatively impacts phase response it is more that if you don't start from a properly summed crossover in addition to the room you now have the added variable of frequency response issues created by the crossover. For modestly sloped crossovers phase distortion does not seem like a major issue, it is possible to use linear phase crossovers to eliminate the phase distortion but that requires a lot of processing power at low frequencies and has other drawbacks.
Of course YMMV, everyone has different rooms, speakers and listening preferences. I find integrating subwoofers as described above works well for me, it may not be the best solution for you.
Michael
Okay so apparently my mains follow a BW2 at 108 Hz, I know a BW crossover does not sum as flat but rather as +3dB at the crossover frequency. So what's the next step from here?
Also is it normal for a 6.5 inch woofer's HF roll-off to start so early? or am i missing something here?