What do Oil Caps do and how do they effect sound?

Thanks for all the wonderful response. It was a pleasure to read them all. This will definitely help me understand my new Cary tube preamp and also help me when deciding on a new amp and weather to go SS or Tube when the time comes.

Thanks for the correction Ed_sawyer. My bad for quoting the dealer without double-checking the info he gave me.

Bombaywalla, how did the ASL sound?

Brooks1,

The sound was exemplary, if I may say so!
The sound was very transparent, plenty of high freq. & low freq extension & since I was listening to single-driver speakers (Cain & Cain Single BEN & Carolina Audio JTM) the sound was so very coherent i.e. music had very good timing! With the ASL SETs, the midrange was additionally magical.

If you have the inclination, I did write a lengthy report on my ASL/single-driver experience. It's long, so have been warned! :-)
http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1072830036&read&keyw&zzsingle=driver=speakers

They smooth out the sound (more refined sound). Given more details and seperation, music notes have more decay and body.