That printer should give you really bright and intense colors on EQ Printables. I assume you've run a nozzle check to make sure there aren't any clogged nozzles? (It's always good to check the obvious first.)
I haven't yet had a chance to watch Kirby's suggestions, so these may be a repeat. The first thing I would try is upping the saturation. What I usually do is set the "Color Management" to "Color Controls" in the printer driver. (You get to the printer driver by clicking on "Properties" from the print preview window in KK.)
Once you have chosen Color Controls, you should see an option for Color Mode. I think the default is Epson Vivid, but I always set it at "Adobe RGB" and Gamma at 2.2. (I'm on a PC, so that Gamma setting may need to change if you're on a Mac.)
Hopefully you'll have a "Settings" button - or a tab or something - to get you into the Color Controls. This is where you can increase the Saturation. I often put Saturation as +15, but sometimes I put it at +25 depending on the image. I also usually put the Contrast at +5 or +10. I don't usually play with the Density, but that may help put more ink down. I'm guessing that it may make the print look muddy though.
Also, I choose "Premium Presentation Paper Matte" for the paper type and "Photo" Quality.
You WILL need to experiment unfortunately. (I do for every series of images I print.) I know it's hard not to think about the cost, but really you just need to "let it go". I learned that lesson early on - if I didn't, I would never be where I am today.
That said I do print smaller images and move them around the sheet just as you said - and I make notes under each print out so that I know what changes I made!
When you ask about changing settings in Photoshop - are you referring to adding adjustment layers to the image itself or making changes in the printer driver?