Passive pre-amp in a cable?

The conventional wisdom on the value, I believe, is 1/10th of the imput impedance of your power amp (which you don't state). However, I've used 10K ohm passives with Pass Aleph 3 power amps, input impedance 23K ohms, and had no trouble. A bigger factor is the quality of your CD player output stage. I'd be inclined to try passive operation befor cutting up any interconnects. Get two pairs of RCA jacks and four resistors, a good brand being Holco. (The Parts Connection can supply all these items, as can many others.)
Build a simple fixed attentuation network: if you want to know how to choose resistor values for various attenuations, contact me and I'll say more. This experiment will tell you whether your CD player is up to being used with a passive preamp, and if so, whether you like the sound. I very much do myself, and will never own an active preamp again (except for a phono preamp for the tiny signal from a cartridge).

Kitch, I'm afraid you need a genuine expert on this, which I'm definitely not. I'm not surprised by your great sound since my 10K ohms passive sounds great with 23K ohms input impedance of my Pass Aleph 3's, that ratio being far from 1:10. I'd leave it alone until you feel the itch to upgrade (assuming you do feel such itches), then replace the pot by a ladder stepped attenuator and go for 10K ohms when choosing resistors for it. Or you could get four RCA jacks and four resistors, two each at 4K ohms and 6K ohms, say, and solder together a test rig (you could use the resistors two ways, either 4K as Rs and 6K as Rg or the other way around) and see if it sounds better. Of course taking the pot out of the circuit will help the sound, ruining the test somewhat, but a qualitative difference might still show up.