My apology for the late reply, I forgot it after sending it to the "reply later" queue.How does the 18Sound compare to the D2430K, for measurements and for sound?
Here's how they both measure on-axis with no EQ on the M2 waveguide - 4015BE and D2430k;
As we can see, the 18Sound isn't optimized for the waveguide with more wiggly behavior in the upper range, but has considerably more output lower in the range. (Below 1kHz isn't really fair here because the 2430k is used with a passive filter with a highpass function involved).
Perhaps some of this uneven response with the 4015 could be solved by experimenting a little with different washers when mounting, but I can't be bothered.
I can't find distortion measurements, but if I remember correctly there was about 15-18 dB less distortion in the 4015BE around 1500 hz.
How does the sound compare? It's just more impact combined with a more relaxed mid the 4015BE driver. Smoother and more snap/punch at the same time, if that makes sense? Anyways, for a DIY'er I would probably recommend a JBL 2451 with or without Aquaplas as a sensible middle way between the pair.
Or cutting off the flat edges of the M2 waveguide and combining it with a good smaller woofer to cross over in the 1200-1800 hz range with the D2 driver.