On your 7894, have you got the MX button in the bottom left corner of the unit. If you have and this is turned on, then TURN IT OFF !!!
Hope that makes the sub louder. Also, go into the tone controls, and change the crossovers and sub level.... Anyway, I think you need a more powerful amp for that sub.. i think that W3 10" needs in the region of 250w RMS - your amp only puts out around 100 RMS (I think) - if you want to stick to alpine, go for the M300 or M500 mono class D amps, or a Diamond Audio amp...