The xp won't be that much lower and it will even out because you can kill faster. You may have to use one of the other characters as you main because I find it really annoying getting the right quests to do if the main has already completed them.

You can also spend a few hours leveling one of your 60s up high enough to quest with your group so when you get to 80 you will have an extra option of what to use. Any class would work well with your lvl 69s.