Can any one tell me if there is any problem using older ampliers that
have
adjustment range of only 0.1 volt to 2 volts on the gains with a newer
head
unit that has 4 volt preouts?

It won't be a BIG problem, but you won't be able to the enjoy the
extended headroom that 4v provides.

A 4v signal doesn't provide extended headroom. Perhaps you meant something
else?