If you were buying from scratch, I'd say a MKII all the way and be done, but 900 bucks buys a whole lot of other stuff that will make you happy as well.  I say keep the MKI, and be happy that you pretty much have the best SS amp there is.  Chasing that last 1% is a waste of time and money. 
  
I know it's not the HC way, but it's the best advice you're going to read.