Help for a "NEW" tube owner

Hello fellow tubeheads. I try to participate in these threads when I can offer sound advice, now I look forward to your help. I just replaced my Krell KRC-HR with an Audio Research REF TWO. WOW! No this is not the first time I have heard tube gear I generally never cared for it before. This is the first preamp to sound good to me in my system. Anyway is there a big difference in sound between the REF two MK1 and MK2? Can you describe the sonic improvments? Should I leave it on for best sound?...or will that cause wear on the tubes?...or does it require a signal to do that? What maintenence is required of this unit and how do you know when its time to replace the tubes? How long do they last? ...and lastly can anyone recommend replacement tubes for this unit? By the way I am driving a pair of Thiel CS-6's through a Krell FPB300 and levinson front end. Sorry for all the questions. Benn in hifi a long time, never bothered with tubes before. thanks!