Well, any pre amp isn't passive( in the sense it does control volume) but I guess what you are asking is a pre-amp that just has a good quality volume control. Most pre-amps have inputs that control input from various sources; turntables, CD players , cassette players and etc and the pre-amp effects this due to additional circuitry and etc.
Yes, maybe just a straight line to an amp would be nice but in the real world for most people it isn't viable.
Any really, most CD players have enough output level connected to an amp with a volume controller( or some amps with a volume controller) sound very nice.
In short, a passive pre-amp is just a volume controller.
A passive preamp does not have any active amplifiers in it. It may be an input selector wired to a passive volume control or stepped attenuator. It can also have transformers in it that will add some gain to the signal passively and isolate the sources from the amp. There are different opinions about whether it is better than an active preamp. It basically depends on the system (input levels, amp and speaker sensitivity) whether it will play loud enough without the additional gain that an active preamp will add. Most CD players will put out max 2v. A preamp can put out as much as 10-20 times that much on peaks so can be more dynamic in driving your power amp. The interconnect cables become much more important in a fully passive preamp setup as well.
If the cables are high capacitance they will tend to affect the frequency response and the cables will interact with the output impedance of the source and input inpedance of the amp without buffering to prevent this. Your CD probably is fine to provide this since they would not want it to interact with other components.
What sounds good depends a great deal on what level you are at. I don't mean to dismiss your opinion in any way but when trying to listen to differences in audio it all depends on the quality of the system and the experience of the listener. You need both to be discerning about these things. Even with these it is easy to fool yourself.
Thanks for the explanation and I have good quality interconnects but don't buy super expensive cables, for the most part coax and cat6(DIY cables) works pretty well. I have a decent system(s) built up. It is much better then your normal consumer system but not super audiophile quality. I generally buy used and trade in the old stuff for full value what I payed to upgrade. At least that how it works with the stereo store that I buy from.
I have a mix of vintage and newer equipment. I don't really want to pay for products like Wadia which do offer better sound but I just enjoy the hobby of trying different equipment.
Listening to high end systems is a good experience but the value of going to a mid HiFi to super Hifi is a bit much!