I've had both so I hope this helps.
The 640A IS richer and more substantial sounding than the 740A. The 740A is a more refined and analytical sounding amplifier that can seem a little on the lean side in some systems and in some music. I've had two 740As and both times I've ended up selling it because I've found it too clinical. Its a great amp in many ways but I won't be going back there again. It does seem to lose some of the joie de vivre that the 640A brings to the party.