You actually are always printing at 2400*1200 DPI, because that is what the printer does by hardware.
What you see in the driver is how the jobs are interpreted and ripped.
So the Driver takes your job and renders it according to the resolution
And here is where the real difference is noted. 600DPI does not equal 600*600DPI, they are entirely different measurements.
600DPI is the amount of dots used to make a digital image appear on screen total within one inch.
600*600 is very different, in the CMYK world, unlike the RGB, you divide resolution by the toners.
So with this CMYK device you have the 4 colors, so you divide 2400*1200 by 4.
So each color (CMYK) prints 600*300 DPI, when all 4 colors are laid down, there are 2400*1200 DPI.
You can modify the ppd using TEXTedit, but you will almost certainly get worse results.
Take a pdf and print it, then dump the same pdf into EWS (the webpage in the printer) and see if the output matches.
If it doesn't, then you troubleshoot application/ppd. If this were a Windows/Mac thing I would say you need to enable Let printer determine color and maybe Simulate Overprint in the advanced settings in Acrobat/Reader. But I don't at all specialize in Linux/Unix.
The CUPS ppd file for Linux seems to be autogenerated for the Phaser 6510 but it has the wrong maximum resolution so I'm concerned the reason my prints look a bit poor is in part because the maximum resolution of 2400x1200 isn't being selected.
Can anyone say why it happens like this and guide me in making the ppd get generated more correctly, especially in this respect?