The short answer to your question is that it is ok to leave two channels unused. That being said, bridging doesn't automatically mean that you will blow the speakers ... you can simply turn down the gains /shrug.
Besides that, how do you have the mid and front speakers wired? That amp is rated 100W x 2 @ 2ohm, so if those speakers are wired in parallel they should be getting 50W each. Do you find that they are too quiet? Why not just turn down the gains on the rear speakers to blend?

 

One more question. I do not know what the gain is currently set at on the amp. Should I have it turned all the way up to get the full 70 RMS watts that is advertised?

 

Thank you for that Destarah, that is very helpful.

I didn't really know the difference between parallel and series but just read about it and I see the speakers are wired in parallel.

The problem is that yes, the speakers are too quiet. I don't want to turn the rear speakers down to balance it out as when we are driving, the wind overwhelms the music. I would really like to make the system louder and I think the best solution would be to pump more watts to the front speakers since they are getting less than their RMS setting as is.

 

Also add a pair of phono (RCA-type) plugs to the unused channels so that it doesn't oscillate.
Sometimes the amp/circuit may have a weak resistor in the front end but this one is strong crow-bar type.

 

If I had six speakers and a 6 channel amp, I would likely be asking where.what wire to use to wire pairs that are in in series or parallel to be wired on their own channel.
Not asking about using 4 channels for 6 speakers.
I think the question could be wrong?

 

I have 6 speakers and 4 channel amp. I am asking how best to hook up a second 4 channel amp.

 

@Holmz - the OP currently has a 4-channel amp and is using it to power 6 speakers. He is looking to install a 2nd identical amplifier so that each speaker can be powered with it's own channel. I took a quick look at Infinity's website and they do not have a matching 2-channel version of the ref-M704a /shrug

@andysam - something I should have mentioned earlier, would you consider selling your current amp and replacing it with a 6-channel? JL Audio makes a 6x75W marine amp ...

 

JL Audio makes a 6x75W marine amp

I thought about getting a 6 channel amp, but they are over $600. I can get another 4 channel Infinity for $180. I doubt I could get even $75 for my used amp. Plus another 4 channel amp gives me the flexibility to add 2 more speakers in the future, or one small sub in the future.

 

fair enough, Crutchfield shows the M704A @ $350 and the JL M600/6 @ $640 so if you can swing a M704A for $180 that's a no-brainer
Having spare channels for future sub is a brilliant idea (don't weigh anchor without a sub!)

 

I'm not sure about the class D amps, but you never wanted to leave open channels on the class AB amps from back in the day. Here's how I would handle your situation for now (and change over later with another 4 ch amp when I got my sub/or 4th set of speakers). Change speaker order to run middle set of speakers on front 2 channels of amp, run fore and aft pairs of speakers in parallel (rf and rr together/ lf and lr together). that will help balance out your system overall. Then to set the gains on your amp- Start by turning down all the gains all the way, fade HU all the way to front, turn main volume to about 3/4 up, slowly increase gain on front channels on amp until you just start to hear distortion then back in down very slightly. fade all the way to rear and do same steps to set rear gains on amp (fore/aft speakers). return fader to center and tweek fader only if/when needed. Keep max volume on HU limited to three quarter (the radio itself starts to throw out distortion above 3/4 even through pre-amps) and you should be golden until future upgrades.

 

Forgot to mention, adjust gains on amp with boat running if possible (I know PITA right now) as alternator puts electrical system @ 14.4 volts as opposed to 12v battery and will increase the amp output slightly. You can get it pretty close for now and just tweek it when you're back in the water next spring.

 

Yeah, didn't read that last sentence until after I hit post.

 

Also apparently forgot how to spell "together"

 

Public Service Announcement - it's possible to edit your posts

If you are just looking to boost your post count feel free to ignore this PSA ...

 

Not what what I was after. Thanks for the heads up on editing, now if I can figure out how to delete the last two I'll look (slightly) less like an idiot.

 

I am, however, going to revise my advice on setting amp gains by ear. I'm of the "My car(boat), my radio and I will bite you if you touch my buttons" school of thought. My radios never go above 3/4 volume, so I can get the cleanest, loudest volume available. If there is ANY chance, accidental or otherwise, that it will get turned up over 3/4 vol. then the amp will clip and you'll be buying several new sets of speakers. To play safe, especially on a boat (wear your lifejacket), follow steps in previous post, except have the head unit at MAX volume when setting the gains. If your ears (or mindset) aren't (isn't) able to perceive (chooses to ignore-see "mindset") speaker distortion, then have someone else listen while setting your amp gains. It takes very little amp clipping/distortion to completely trash a speaker regardless of price or quality. And I would snag that M704A for $180 and just put it on a shelf until I was ready for it- great $.

BTW, if anyone can tell me how to delete a post, I'll get rid of them (strikeout was best I could come up with). My hands and mind are much more comfortable with tools than keyboards.