How do they compare? How do they sound different?
My current setup is 2 10" RD Alpha's V2s w/flatwind coil option in a 2.5ft^3 box tuned to 36hz.
I used to have a 15" FI Q w/bp power option (actually i still have it, its in the classifieds //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif). I used it in both a sealed (2.5ft^3) and ported (4ft^3 tuned to 32hz).
The FI Q has a slightly more "warm" sound compared to the RD Alphas, although the both sound very warm compared to the AQ SDCs i used to have. They both sound very similiar on rap, but i enjoy the sound of the alphas much more on rock (may have to do w/the tuning though). The Alphas also hit the higher bass notes (~55+) with a bit more authority than the Q. Despite the smaller box and higher tuning though, the alpha's seem (to the ear) to have a more linear response across the frequencies when comparing between their ported boxes.
Build quality on both is top notch, but if i had to choose i would say the Q seemed built slightly better.
They both easily handle their rated power, I had ~ 1500w going to the Q before volt drop/imp rise (PA 2400db), while the RDs have ~ 2000w each from an AB500.1 @ .5ohm. Since i was using diff tunings/sub sizes i can't really compare the output between the two, so i can't really say which would get louder off the same amount of power.
Honestly, they are both great subs, but i prefer the Alpha's over the Q