In my opinion it has to be the cable debate. I know that I cannot always quantify or put into words what I hear with cable changes. I do believe they can and do make a difference, but proving it is another proposition all together.
The differences between
really great vinyl playback and digital are readily apparent, so it is easy to decide which camp you are in. Tubes vs. Solid state? That's a little tougher as the SS designs have come a long way. Still, decided which side of the fence you're on is not that difficult. Same with "neutral" vs. color, again it's a preference for a certain type of sound, like the CD3K vs. HD600. Hi-rez arguments may be more difficult to solve, but to me, still not like cables, or other tweaks like cones.
The un-solvability of the cable debate has more to do with whether a person believes cables make any difference at all, rather than whether they like "warm" vs. "neutral" cables. I have read many accounts of "true" audiophiles (people with gear that is on anyones kick-ass list) who do not believe that cables--beyond the difference between high quality purpose-made and generic low quality(example: zip cord vs. dedicated speaker cables)--make any appreciable difference. I AM NOT IN THAT CAMP...I can clearly hear differences in many cables. That does not mean that I can prove it to someone who doesn't believe it.
Sounds like a familiar argument doesn't it?
Oh yeah, for the record:
Great vinyl over PCM (although, I can't afford that level of vinyl rig)
Tubes over solid state
SACD over DVD-A
Cables DO make a difference
A little color
2 channel over poorly done MCH (haven't heard good MCH yet)
.
