Is there any one knowing the output phase of this amp?
My preamp (Merlin) is a invert-phase output, if the Verdier is non-invert phase output, I should change the speaker cable polarity inversely, right?

What speakers are you talking about?
If you have already changed phase from the Merlin to the Verdier you don't have to change again to the speakers. But, this issue about phase invertion is a very old and long story....because some piece of music sound better with inverted phase, a great part of the recordings are recorded out of phase....
In resume, most of the time it doesn't make any difference....unless your speakers are very polarity sensitive.

Why don't you try it both ways and tell us what you think?! Do you have any pic of that Merlin beauty?