Problems with my Mark V

Dear Srs.

A few days ago, finally my MB Mark V arrived to my home (Chile). I bought this amp from ebay one month ago. A friend of mine bring it to me personally, but he doesn't put out the tubes from the amp when he returns to Chile.

Here, I bought a 500 W step-down transformer and connected the amp and then selected channel 1. Really awesome !... later I moved to channel 2 with a little more gain, and 10 minutes after that the amp loss gain and weight, the same features on channel 3... I turned it off, but later was the same thing.

Is this associated to a damage of tubes, power o preamp??... I want to buy a tubes replacement.. do you thing Mesa boogie are the best? ... have they kits for Mk V?... or should I buy 2 duets of 6L6 GC + 1 5U4GB + 7 12AX7 ??

Please help me .. this is the dream come true but I can not play yet..

Thanks for your help

Is there any change if you switch wattage settings, or if you switch from silicon diodes to tube rectifier?

Thanks for you response. I am out of my house right now, but I read in a mesa boogie file that it is possible change of tone And weakness when you have the effect loop active without anything connected. I tested that quickly And sounds better but I must check slowly everything when arrive to my home.
Thanks again