I discovered that a self-proclaimed audiophile has his CD-Player (18-bit of course ) plugged into the tape stage of his pre-amp.

At first I figured it was because he was running out of inputs, but upon questioning him he said because 'tape stages sound better than CD ones'. I didn't bother going any further apart from asking him if he knew whether or not the tape stage was actually a different stage, or that the pre-amp just switches between the 2 inputs. He evaded an answer.

I can understand that some pre-amps will feature a unique phono stage, but is it likely that pre-amps have different stages for Tape and CD?

Maybe he has a bad solder joint on the CD input?

Both are the equivalent of "line in" so they should be the same, just a pass-through selected by a switch. I am sure it does sound better to your friend, as do the "directional" interconnects he uses. As the signal is AC I wonder how useful directional wire is, or if it even exists without some sort of gate. Oh, well.

There used to be tape inputs on HiFi preamps. Tape requires additional amplification and particular equalization, in a manner similar to phono. That has been built into tape decks for forty years or so, thus no longer included in preamps. If someone had a preamp so old as that, it could no more readily be used for CD than could a phono input.

Maybe it's a tape monitor (which still shouldn't affect sound quality unless the amp is extremely shoddily designed and even then chances are bigger it'll sound worse due to crosstalk from the "source" input).