About the DE250, I think certain crossover configurations are more sensitive to unit-to-unit variations in drivers than others. This kind of crossover would then also be more sensitive to shifts in characteristics at various power levels.
I have not noticed any changes in the DE250 beyond the normal unit-to-unit variations.
About mods, there's nothing "sacred" in my designs but they have the benefit of being mature and well-tested.
Minor dimensional changes don't usually hurt anything but they can change the position of standing waves. This can make anomalies in the midrange. So many DIYers think of the cabinet dimensions only in terms of the Helmholtz cavity resonance and forget about the standing wave resonances. Higher frequency standing waves are damped by the insulation, so in small cabinets, you sort of can disregard them. But in large cabinets, the standing waves line up in the lower midrange where the stuffing is less effective at damping. So position becomes important.
The four Pi has another complication, which is that it is pretty tightly packed. So when people start talking about changing it a little bit, they often find they can't do it without changing the shape and/or position of the port too. By the time they're done, it's really pretty different. The standing waves line up differently, the positions of the driver and the port are different with respect to the internal cabinet boundaries, so the nodes line up differently. I've seen 'em create a big peak in the midrange from what would seem to be a pretty harmless mod.
Some have asked about making the cabinet larger, to increase bass response. My reply is usually the same. I'm not concerned about Helmholtz resonance - That's the easy part. Make the box larger and you gain extension, up to about 5ft3
. But again, we move where the standing waves fall. It's like room modes, really, except the range we're concerned with is the lower midrange.
I know of only one mod that has been tested, and so naturally I'm comfortable with that mod. It adds extension by larger size. If you use subs, it isn't necessary, but if you don't mind the extra size, it is worthwhile. Extension is gained, and it doesn't create ripples in response. So it's good. But other mods really should be verified by acoustic measurements. If you don't have a way to perform measurements, my suggestion usually is to stick with the stock plans. Not because it is sacrilege to change, but because you cannot know what you have.
This and more information can be found in the Pi Speakers FAQ