Has anyone experienced like me a degradation from the phone jack of those remotes ? For that reason i never used them , i'm even thinking of by-passing the volume control and its nasty electronics... What do you think about it ?

It is probably oxidation of the volume potentiometer and 3.5mm stereo jacks on the remote control. Try giving them a clean and see if it improves.

No , i mean it sounds "fine" but the sound is less good coming from the remotes even with newly boughts walkman , i've always noticed that or do i have bionic ears ?  

See, you have a jx505 , we share the same remote with my 303 , did you ever heared a difference between from the remote and from the unit itself ? May be it improves the sound of the 505 !!?? That's (must be) it ! (little joke...)

I have just dug out my JX505 and its remote controller. The volume controller on the remote controller was a bit crackly but otherwise comparing the JX505 with and without the remote controller, the only difference I could tell was that the sound was slightly quieter when using the remote controller. The dynamics of the sound had not changed. If reasoning that the JX303 has capacitor issues, then I suggest when replacing the capacitors to give extra attention to cleaning the circuitry around where the audio path and remote controller paths are ensuring there are no short outs due to leaked electrolyte.

I have a PX10 which had horrible dynamics. It sounded like I was listening to medium wave radio transmission. It turned out to be leaked electrolyte shorting out across all channels of the headphone jack underneath. After cleaning that part of the circuitry it was sounding much better.

I have always noticed that for all my devices having a volume control feature on the remote , you said it , you've noticed a slight difference but it exists , may be you feel comfortable with it but i do not. As i said this is not a recent issue , it's always been , not only today as a granpa(the walkman, not me!).The slight difference is annoying to me ,i want the best sound possible,that's why i'm thinking of by-passing this. There is no caps in the remotes ,no leakage no nothing like that , the sound coming from my 303 is fantastic , i doubt the remote is using something else but the fantastic sound for the...sound. When i say "degraded" i mean a sound less good (not as good) but nothing like a technical problem. I must have bionic ears.