I was originally leaning toward the Fi X. It's definitely a good looking sub. The SA seems to possibly be a little better parameter wise, based on my knowledge (anyone feel free to correct me if I'm wrong here). The moving mass of a D2 SA is substantially higher than that of a D2 X, but the Bl is also substantially higher so it should have the same, if not more control. Fs is similar, within a couple Hz. Fi doesn't publish the inductance of the coil where Sundown does. I'd imagine this has to do with Sundown using shorting rings on the SA to help with inductive distortion. I'm guessing the X doesn't have shorting rings. The SA should play cleaner higher than the X because of that, not that higher frequencies really make a huge difference with subs. The X has a slightly higher Xmax, 20 mm vs the SA at 19 mm. I highly doubt one mm is going to make any sort of difference. The X handles more power, according to manufacturer's specs. It's "good" for 750 while the SA is "good" for 600. With the size of the voice coils (3" on the X and 2.5" on the SA), I'd venture to say that both can handle significantly more than rated. That being said, I've got 1038 RMS @ 2 ohms assuming I get a clean, consistent 14.4 volts. That's where my MRV-M1005 bench tested. I've always been afraid that playing too far below a speaker's "rated RMS" will push it no where near it's capabilities. I guess what I mean by that is the power that goes into a speaker has an exponential effect on the driver movement. 500 watts on a driver made to handle 500 watts will be louder than 500 watts on a driver made to handle 1000 watts. I don't have any evidence to support that statement, but for whatever reason, that thought has always been in the back of my head. Lastly, my box is 2.5 cubes sealed, roughly. That falls right within the X's recommended box sizes for sealed applications. It's ever so slightly large for the SA. Now, if you've stuck with this post until here, you've pretty much read everything that's going on in my head related to this decision I'm trying to make. Knowing that, what's your input? Again, thanks in advance for any advice or what have you.