Mcintosh C2600 preamp tube upgrade

The C2300 enjoys a great reputation. I hope time will put the C2600 in the same positive light. 
I only upgraded the line stage because I only have CD player for source equipment.
Im curious about your thoughts on burn in time. Some say as low as zero hours  just warm up for 10-15 minutes prior to use, others say as high as 200 hours.

The input about the tubes is great, thank you everyone.  A couple of further questions about the 2600.  Does anyone know if the tubes are utilized when going through coaxial or optical inputs vs the balanced or unbalanced inputs?  I clearly hear a difference between the various inputs.  Also, are you enacting the tone control on your inputs.  Again, I hear a difference.  My components are the C2600, MC302, Martin Logan Summit X speakers and a Marantz SACD player.  Any thoughts / guidance is greatly appreciated.  
Thank you!

I have a C2500 and in that one, though the tubes are identified as "McIntosh", they are JJ tubes from the Czech Republic.

As my preamp is under warranty, I'll hold off on replacing the tubes, when I do, I've had several people point me to the Genalax Gold Lions as a great upgrade.

Interesting, I did not know that there was a 12ax7a.  I use 12ax7's in my non Mc preamp.

Jetter,
I found somewhere on the web that the "a" at the end of 12ax7a meant that it had a wound filament.  Not sure what that means to the sound quality.