@lak "As audiophiles we should realize that one always drops a hugh chunk of change for a marginal return." WOW! I'm sorry to find that to be your audiophile reality. But gee, speak for yourself , not the rest of us. In no way do I feel that my investments in improving my gear have yielded "marginal" results.
Examples: I took an $1800 line stage and over a series of modifications, have for a total expenditure of $4000 have a superb, reference quality preamp.
As previously mentioned, the Modwright transformation of my Marantz SACD player has enabled it to provide fantastically musical sound from silver discs.
Most recently, I sent my previously modified McCormack DNA-1 power amplifier to Steve McCormack at SMc Audio. This "20th Anniversary Upgrade" as I like to call it included a new Furutech IEC inlet, new Cardas RCA jacks, new speaker binding posts, new soft recovery diodes, an entirely new input board and all new heavy gauge wiring. All of this for $1300!! As I was warned by Steve, it took about a month to really sing, but sing it surely does.
In summary, IMHO, buying well designed, well built equipment that you can, over time improve upon with carefully selected modifications, can yield an amazingly musical system with money well spent, minimizing the "law of diminishing returns".
One caveat... If you do go this route, you must vet the modifiers very carefully. Have several extended conversations, listening carefully to how the prospective modifier discusses working on your equipment. The top notch modifiers will talk more about you, your system and what your goals are for the sound of your system. If the conversation is mostly about them, especially if it includes bashing others in the industry, you are probably best to look elsewhere.