I always keep the cap off on tubos when i am storing them. some people say they can still "breath" with the caps on while others say tubos can hold humidity for awhile when shut. i personally think tubos where made to hold humidity for a short period of travel and i dont think they breath as well as cello hence why i leave the caps off but that's just my

my guess would be leaving the caps on extremely slowed down the aging process and depending what RH you store at they may start to be getting to dry