I hope this thread will inspire more to boldly go the extra mile. For my system and room, it would be much easier to list what I did not modify or build or design myslef.
Well, that list contains 0 items - I have nothing "stock", all the purchased items are heavily modified, and are in strong minority compared to what designed and built.
One thing I would like to point out is the room treatment: walls are treated with the triple system of: mass loaded vinyl - green glue - quietrock drywall. This is the method used in movie theatres to isolate one room from the other. Each layer provides isolation at different frequencies, the end result is astonishing, provides approximately 65dB additional isolation for the walls.
Here in the US, most homes are built with absolutely no respect for sound-proofing, and in every single apartment (here in HI) this has been a vital issue for all my friends systems. The soundproofing allows me to play music at any time and at any volume. Pretty sweet as my neighborhood has strict quiet hours policies.
The difference is not just for loud music. It also dropped the noise floor tremendously, which allows listening to music more quietly with more enjoyment, and also listening at super low levels.
The third HUGE advantage of proper room treatment is DEEP BASS. Finally, I can enjoy not only deep bass, but infrasonic bass - while untreated homes are basically midbass sources, have problems reaching down to deep bass, and not a chance for infrasound. (My room supports down to 13Hz).