New power tubes lacking low end and slam

Quick question: I installed new KT120 power tubes (same as the prior original tubes) in my Cronus Magnum II for the heck of it. Previous tubes are roughly 3-4 years old. I had not been having any problems with the original tubes but figured fresh new tubes couldn’t hurt. After installing and biasing the new tubes they sound weak to me. Not as much low end and not as punchy. Is this normal? Will the low end improve after burning in? Or should I throw the older tubes back in? 
Thanks and happy 4th!
Paul