How should I install the new tubes in my stereo amplifier?

Matched pair in each channel. 

Well I don't know how your amp works either. But I assume two things, 1) it is a pp el84 amp and you have to bias it manually. Some amps have a bias adjustment for each tube, some have one bias adjustment for all of the tubes assigned to one channel. 

If you have the ability to bias each tube individually then even  using 'matched' tubes is just a bit of frosting on the cake, so to speak. A good thing, but not essential.  

 If you have only one bias pot for all of the tubes dedicated to one channel getting matched tubes is essential. If you split up the pairs you would no longer have a optimal bias setting in either channel. 

Its a wild guess but I think your amp has a bias pot for each channel. If that is so what is really essential is to properly bias your tubes initially to the manufacturer's recommended setting and thereafter regularly check the setting to insure they are operating properly. If you find a tube bias' drooping and/or you can no longer bring it's bias up to match the other tube in the same channel you will probably find it beneficial to replace the tube.

Hope that helps a bit.

 

Correction -

1st sentence, last paragraph - 'each channel' should be 'each tube'.