Re: 1998 250 DA Stereo question - amp and subs
Eric,
More info is needed.
What are the specs on your speakers? (RMS & peak power)
Does your existing head unit have pre-amp outputs? If so that would be the easiest way to add your amps and subs. I would use two amps, one for the four "up top" speakers, and one for your subs. If you only have one set of pre-amps you can daisy-chain the amps together on some amps.
If you don't have pe-amp outputs, you can buy amps that allow you to use "speaker level" inputs. The sound is not as clean, but hey we're talking about a boat, right?
Your friend was right, I would use the headunit to power your cabin speakers. Another thing you may consider if you spend nights on the hook, install a switch to cut power to your amps so that only your cabin speakers will play when you want quiet time. All you have to do is install a switch on your amp switching wire from your headunit.
Oh and one more thing. I would run better wiring to our existing speakers to handle the extra power.