comeelec - Hi and welcome to the R3A circle.
I must admit I was surprised when I read that you were getting brightness from the equipment you listed. I've always found the Planet to be very warm and 'analog-like' in that Rega sort of way, with a slightly recessed upper midrange and treble. Based on what I've heard, I can't think of any other cd player that would 'fix' the issue; in fact the Planet would spring to mind as a recommendation if you had a different cdp. I suspect the problem is elsewhere.
I don't know your amp, but I've heard 300Bs that have a midrange/treble forwardness (not brightness exactly, but sort of...) even while being warm/luscious in the rest of the frequency range...
I'm not familiar with your cables, but that might be a place to experiment. Van den Hul might work well in your situation; I've also heard that MIT cables can help tame a wild top end, although I don't have any experience with them. I would avoid silver cables in your situation.
As has been mentioned elsewhere in the forum, the vague consensus is that, if anything, the De Capo i's are slightly/greatly more refined and less bright on top; however, if you're coming from another speaker, it's likley you're not used to hearing as much high-frequency information. I know that I always think other speakers sound laid-back on top compared to my (non-i) De Capos.
I agree with mc, though--your room is a likely culprit. Do you have wood floors? plaster walls? un-damped reflection points? A good way to test (very unscientific, for you number-crunchers out there) is to sit very close to your speakers, and listen to how the tonal balance changes as you move away. If things sound 'right' when you're close to the speakers, room acoustics may be fouling things up at the listening position.