Personally I don't like the sound of the 905. A bit hollow sounding. Not very good extended lows. Haven't heard the yamaha, but since the tops are rcf, I would stick with rcf. I do know the 8004, as I have several of them. Killer sub. Used them several 100's of times now.I have heard the 8003-2, and it is also very good for the prise. I only heard then twice, but can recommend them as a budget choice.Both will produce dry, defined lowend. The 8003 has a stereo input, the 8004 hasn't, so 1 8004 would be a hassle to integrate in your system. I also owned the rcf hd32 mk4, which is almost the same as the 732. The Hd32 will outrun a 8004, if that is of any concern to you. They coupled beautifully. 1 or 2? If I were you, I would start with one 8003, and get a second one when budget allows. If you can swing the extra money, get 2 8004. They are worth it. Maybe second hand is an option? Also try to find a dealer and talk to them. You might be suprised what they can do with pricing. Get the covers!