Can someone explain

I lost a tube in my Cary SLP98L last night. Actually, one channel has been fading away for some time and I just figured out that it was one of the SLP98's line stage tubes. I replaced the line stage pair with spares and all is well again.

I am wondering about the second pair of tubes, though. The so-called "buffer cathode followers." All four tubes are the same age (close to 8 years of use) and I'm thinking I should go ahead and replace them as well. I know from experience that the line stage tubes make a big difference in the sound of the preamp, as they are actually carrying the signal from my source, but can someone tell me what the "BCF" tubes do, and if they also have an important role in the sound? I have one pair of NOS VT-231 tubes left and don't want to use them in the buffer position if they don't make a major difference.

Hmmm, thinkin' that I should dig out the Chinese tubes (maybe Ruby?) that came with the SLP98 and see if they make a difference I can hear in the buffer position....

Still, if anyone can explain the function of the BCFs to me I'd be grateful. A link to an explaination of tubes for the electronically challenged would be nice also, if anyone knows one.

Thank you!