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!
Welcome to the Tube World !!!!!!!!!!
Hope you have another Big WOW when you change your Krell FPB300 to a high quality tube amp !!!!!!!
Just check out AR's website on the sonic differences between the two.Upgraded my LS25 to mk2 and the difference was far from subtle.AR claims the REF upgrade is similiar.As for leaving the unit on,I personally would not unless you like replacing tubes.After about an hour in warm up mode,is recommended for listening.You may want to consider upgrading the stock cord as you will find the improvment in sound to be very cost effective.......
Another thought:

Rather than bump your Ref2 at ARC, have a listen first to the BAT VK50SE. In my system, it left the Ref2 cold.
Victor Khomenko at BAT is the MAN when it come to implementation of 6H30 tubes.

I must say though, ARC is fairly conservative and a great company. Now that Johnson at ARC is is jumping on the 6H30 bandwagon, he must feel there'll be a good supply of this tube for some time, which is good!

Thanks for all the help! I think I will upgrade to the MKII.
The upgrade is costly, but you can assure that there are a good supply of 6H30 tubes out there. But if you can get your hands on some NOS (New Old Stock) tubes. It might be a cheaper alternative. Many of my peers are having excellent results using NOS tubes in their LS-25 and Ref 1/Ref 2 preamps. According to several friends, the result is not too far from the upgrade. This is the route I am going as I have collected quite a bit of NOS tubes to last me for a long time. Definitely get an aftermarket powercord for your unit.