Experience with AMR DP-777

Hi there, I'm fairly new to audiogon, but have been assembling high-end systems for about 15 years now. Recently bought an AMR DP-777 and swapped tubes and it killed my left channel the first time. I'm no stranger to swapping tubes (in DAC's and Preamps), so I was surprised this happened. I've read some other threads that this sort of stuff has been an issue with this DAC. Of course the DAC sounds, but I have two questions/points:

1) for those of you that have experience rolling tubes with this DAC, what is your procedure to make sure it does cause some any system failure with the unit?

2) second thing is I've sent the unit in for the SE upgrade and will be able to report back to you guys the difference/improvement in sound, as I know it's sonic signature very well