Ahhh, the age-old question... What do you replace your stock speakers with.....
It is as if somehow universally, stock speakers are bad or somehow substandard. I understand this thought process, as we all know our stock speakers in our cars can be improved upon. We also know of "that" boat that has the bare minimum audio system with the cheapest of cheap little throw away white grill speakers...
Historically boats DID come with very little in the way of audio and the speakers WERE bad, (so was the radio!). That is really not the case anymore...
Old habits die hard, so it is logical that a new boat owner would want to replace the most visible part of the audio sustem, the speakers. However with the stock Kicker speakers running of the deck power only the first logical choice for an upgrade is to get an amplifier for the in-boat speakers.
I am going to go there; your stereo shop may be just trying to take money out of your pockets with an easy sell. Replacing speakers is quick and easy. Adding an amp is more work. They also simply may be sharing a personal bias against Kicker; it is hard to say for sure.
Your stock speakers are seriously good speakers with large powerful motors, sealed drivers, substantial and solid grills. They were designed from the ground up as marine speakers. They meet or exceed several standards for outdoor environmental exposure. Like any good speaker, they will sound way better with some power behind them.
Consider the Kicker KXM400.4. It is a marinized full-range, Class D 4-channel amplifier. It will deliver 50 to 75 watts to each speaker, depending on the actual power output of the amp you purchase. You can find this power rating on the "birth certificate" included in the amp carton. This will be somewhere between 3 and 5 times the power output of the head unit only. You WILL hear a noticeable difference with the addition of an amplifier.
Hope this helps...
Phil
Kicker