Strongly considering replacing the two 6x9 stock speakers in the cockpit of my formula with Polk MM691's. I plan on leaving the kenwood head unit in place. 50 amps per channel, or am I leaving alot to be desired on the table. Should I amp the new speakers?

Any dealers for the polks on this site, would love to give them $$ instead of MiChongDung on ebay/amazon. No good audio shops in local area.

LC

Polk momos are pretty good bang for buck but unless you plan on adding an amp later you might be overbuying. Regular 691s will sound great powered off the deck. Your deck is more like less than 25watts rms btw.

Yes 25rms 50 peak IIRC.

What size amp is needed to fire the MM's to the max capacity?

No head unit amplifier puts out more than 14 watts RMS. I don't care what they claim for wattage, they all are chip amps and they will not put out more than 14 watts.

With that said I would not run any decent speaker unless I had a real amp to run them. You would be suprised how much of a difference that it will make. Get a 100 watt 2 channel amp, it will be the best money spent.

Agreed, the db691's will do the job and free up $$$ for a nice amp. I've used Polk speakers in my last 2 boats, never any issues and sound great. I also purchased Polk's 5 channel amp to power everything, which I hear is no longer available.
2-db691's, 4-db651's, 1-MM1240 dvc.

Are the db691's rust proof? May do 4 and a 4 channel amp. But how much rms watts per channel is needed. I think they are rated 100rms and 300peak. So will a 4channel amp with 50 watts rms do fine or will it need to be closer to 100 watts rms.

Depends on the amp. If you go with a quality amp, say alpine or JL, the amps will have much better spec sheets/ratings. I would rather have a quality 50w per ch rms than a cheap 75w per ch rms. I know MTX was famous for underrating their amps, they would rate them at 12v supply and the output would be substantially higher at 14.5 volts. I would say you would be good with a quality 50wrms per ch amp with 4 691mms or standards...it should actually bump pretty good.

I put the 691s in my Fastech. Big improvement but they are still only 6x9s. You can easily get them to distort with just the power of the head unit. No need for an amp. Jeff Wurl

Look like I will be doing jbl 6004 amp with 2 or 4 691's. I have a huge list of projects this winter and don't want to rush install of two more 6x9's. May use 6.5's instead of 6x9 under rear bench but am I loosing much doing this?

Most likely your head unit was maxed out and it was clipping causing the distortion. Any decent aftermarket speaker can handle way more power than the headunit can supply.