Phase and polarity are very different things-although (sadly) they are misused and interchanged often within our industry.
What is correct for YOUR system?
It depends on the particular cabinets, the crossover points, delay times used etc.
The CORRECT answer is one that has the smoothest/flattest PHASE response around crossover.
So that could be IN or OUT of polarity. Nobody knows without actually measuring YOUR system.
THERE IS NO correct answer that fits all situations.
But in terms of polarity, you have a 50/50 chance
And simply flipping the polarity and listening for the "loudest bass" DOES NOT mean it is correct.
It "could" be correct for the notes in the song you are listening to, but in a different song (in a different key) you may be cancellation at those notes.
MEASUREMENT (NOT RTA) is the best way to find the proper answer.