The tubes could be changed every 6 months if you listen a lot or at the very least annually. They're cheap and it's quite audible when you do. Just make sure to properly bias them per Lamm's instructions. Unfortunately, you can begin to hear degradation in the tubes almost within a week. It surprised me after doing this the first time that Lamm doesn't make a point of recommending frequent tube changes in their manual. I suspect most L2 owners aren't aware of this and how audible it is in their pre to not do so.
