I had Poly Planar head unit and speakers in my boat and in my hot tub. All factory installed.....and all way below average in every way. Overpriced as well. Not 45 watts constant power.....peak only. I would try new speakers first. Bigger, if possible. Look for something with a high sensitivity rating. Also pay attention to the specs as to how low they will go. If you do not get the bass you are looking for, at least you will have improved the sound dramatically. A passive bazooka tube may be the answer if your head unit will power it. Crutchfield will let you return it if it does not work out. The amplified version would shiver your timbers, but you have ruled that out.
By the way, bass shakers are made to go under your seat and vibrate to the music. They are not speakers at all.
Another idea (and probably the best)would be an inexpensive head unit with bass management. Crutchfield has lots of those. Look for one with lots of internal power. I would look at Crutchfield's comparison chart and focus on the watts per channel. I like the Alpines, but I think you will find Kenwood to be the watts winner. Good luck.
