I think I will have to get an "SP4 vs ...insert Fi sub here..." post going, but Ill summarize here too. SP4 Heavier SP4 More mounting depth SP4 4" coil vs 3" on N2 SP4 10" spider and higher mechanical linearity than N2 SP4 More long term thermal stability SP4 Sealed capable vs N2 ported only N2 More motor force N2 Inductance ring that balances the inductance swing better SP4 Slightly lower overall inductance than N2 N2 Well... its cool because its Neo N2 It's expensive because its Neo (and constantly increasing in cost) SP4 Slightly higher Xmax (coil limited) SP4 Lower FS for those that think it matters In general we created the N2 as the replacement for the older generation BTLs. We have seen the direction people are going with the BTLs and arranged the N2 specs to be closer to intended applications (huge, deep hitting, daily setups). Then creating the N3 to fill the void for the high motor force applications like extreme SPL or simply those that like that "sound". The SP4 pushes things a little further in the daily driver direction. While the N2s have proven to work well in 4th orders, they simply don't in sealed. The SP4 does just that, and a little better than the N2 in 4ths. It still works great in ported, with the ability to have a heavier peak down low as well. In testing it is a very solid sub that can take quite a bit of abuse. While not needed (none of the original test drivers we built had it) we do have the option for spaced spiders (2 on the top and 3 on the bottom) with a 1" ring between. In this configuration we have hand sewn on leads in between the spiders. In its standard configuration with 5 spiders and the gapped cone, it is a very stable platform (not a triple jointed sub. The cone is spaced about 1.125" from the top of the spiders. This stabilizes the coil better than the standard triple joint and is what we are doing on the standard versions of the SSD, Q, and BL drivers now). I think the SP4 is in keeping with the times and applications people are heading towards. High excursion, stability, and thermal power handling... albeit at a cost of weight and depth over the N2 series.