May i ask why your friend feels the need to run dual subs?
Does he have a large room that seats more than 2-4 people at a time & have that many people often?
I ask because at his price range, <$250 each sub will not get him that great of performance. Dual subs are almost always recommended because of the smoother bass response across a larger seating area (if placed correctly & carefully). There is a small increase in output as well (3dB if seperated). But unless he regularly entertains a few people, watching a flick, at a time then i would much rather have a single $500 sub over 2-$250 subs. If the $500 sub is placed in the right location, adjusted right & even a little EQed, he could get very good performance that would dig deeper & most likely play louder for 1-3 people setting at the "sweet spot".
Im not saying that for sure he shouldnt get 2-$250 subs. They can have their benefits as well. However, all the facts need to be considered first. Maybe they already have been, i dont know. But i believe a single Rythmik FV-12 would outperform any of those dual subs you listed.
Could you give a little more info on your friends room?
SqFt?
Open or closed area?
How many people regularly listen?
I just want to make sure we guide you in the best direction!