Ok so I made BP6, TH and PH3 as close as I could without decimals.
BP6:
PH3:
TH with a rear chamber as the stub and throat adapter and port as the rear chamber:
So TH and PH3 look pretty similar and both have a tuning around 26/58, but BP6 still has a lower tune of about 24/51Hz, and all of them are 316l +-0.5l.
So it must be the stub between the cone and front chamber that raises the tuning because it is simulated in both TH and PH3, but not BP6.
BP6:
PH3:
TH with a rear chamber as the stub and throat adapter and port as the rear chamber:
So TH and PH3 look pretty similar and both have a tuning around 26/58, but BP6 still has a lower tune of about 24/51Hz, and all of them are 316l +-0.5l.
So it must be the stub between the cone and front chamber that raises the tuning because it is simulated in both TH and PH3, but not BP6.