I had a post and got alot of replies about the lacking bass my WM-D6C seems to have. some say the unit simply lacks bass compared to other walkmans. I did have it fully serviced a while back and the sound is incredible, but lacks low end like most of the other walkmans I have..the shop said everything checked out great using Sony reference tapes for tuning it up. 

considering it's age, is it wise to go ahead and replace all the caps? I did not have that done on the service. would replacing caps make a difference in the frequency response and give more bass? It seems caps have a shelf life and was wondering...

If you want to replace the capacitors, I suggest you use tantalum capacitors, except for those on the power main.
Try to replace the electrolytic capacitors C107 and c 207 of 4,7 microfarad with two  tantalum capacitors of 22 microfarads.

In addition, tantalum capacitors c 135, c 136, c 235 and c 236 from 1 microfarad with 4 tantalum capacitors from 4.7 microfarads.

Let me know if, after this change, the bass response like best.