Congratulations, Dan, not only for the latest improvement, but for the great DIY work you’ve done over the years creating and refining the speakers.
Does the following schematic of the crossover network of an HPD-315 depict the corresponding circuit, at least approximately? https://pinkfishmedia.net/forum/proxy.php?image=http%3A%2F%2Ffarm4.static.flickr.com%2F3455%2F396944...
And if so, does the 50 ohm resistor it shows correspond to the resistor your post refers to?
If so, as far as I can tell it appears that when the switch the resistor is wired to is in one of the positions that put it into the circuit it would interact with the 4.7 uF cap so as to affect the point below which low frequencies are rolled off, in the path to the high frequency part of the driver. With that point also depending on which tap of the inductor is being used.
Perhaps Erik will comment further, as he is especially knowledgeable about speaker design. But if my supposition is correct I’m not surprised that the change from 27 ohms to 47 ohms had a dramatic effect, as it would affect the frequency at which 3 db of rolloff occurs in that part of the circuit by roughly a factor of two, depending on the inductor tap that is being used. Phase shifts at a range of frequencies, probably in the mid-range or upper mid-range, would also be affected.