Vishay resistor break in?

I just loaded my Atma-sphere MP-3 with nude vishay resistors. Should I expect them to improve with some break in? Any idea how long? Thanks
After about 10 hours treble will open up and bass will tighten. Most of the break-in is finished by 50 hrs. Good choice!
Dgarretson is right. The first time I replaced stock resistors with Vishays (S10) and turned preamp on, the thing sounded so bad I almost took them out. About a week later they started to sweeten and open up the texture. The noise floor drops and everything gets better. Be patient.
Thanks to you both. Sound is thin and tinny, but I will be patient. 50 hours by next week:).