I've been tinkering with a used Arc Moto 600.4 I picked up and noticed this little paragraph in the manual:

There is no correct gain setting. Because speakers require
different power demands to reach the same output, the
gains most often need to be used to compensate for these
differences. If you tried to set all the gains at half way you
would probably find the system didn’t sound very good.
Using good judgment and listening carefully to each speaker
is still the best way to tune a system.

I still think if you are sticking with the PN4.1000d 8 ohm speakers are a bad choice if you only have channels 3 and 4 to work with. With the 4 ohm Mmats 601.4s I have gotten all the volume I have wanted out of channels 3 and 4 on that amp from the lowers. In just tinkering with them to see what they would do I had them so loud they would move the hairs on the back of my hand when I put it down there. I understand working within budgets though. Maybe you could sell some of the speakers you find you don't want and try a set of 601.4s.

 

PS Weeboy just saw a good deal on marine treated Mmats 601.4s on the BST forum in case you are interested...

 

 

Sorry that is the Buy Sell Trade forum. It is a sub forum of this audio forum. When you first load the Audio forum you will see the link up top:

Motorcycle Audio-Buy/Sell/Trade