[MORTIS]

I was right there with you until you asked if you should run the variable out at 100%. Running the variable out to your C2500 means you're running 2 preamps in sequence; the one in the 301 and then of course into the C2500. I'm not sure why it would sound clearer to you but I've never heard of anyone using the variable out on a CDP into a separate preamp. Then again, if your preamp does not have the option of volume matching between sources (I believe the C2500 does), then using the variable out will allow matching.

The variable out on a CDP is generally used when running the player straight to a power amp, thereby using the preamp in the player, which saves having to buy a separate preamp. Some folks prefer the sound of that setup, and some prefer a separate full featured preamp while using the fixed out from the player.
If you end up preferring the sound running both pre's then, well, whatever works for you.