All of the amps I mentioned have an input impedance of 100k Ohms, and will match well with the Calypso preamp's 1k Ohm output impedance. They will all drive your Sehring 703s speakers without any problem.
The Belles 150A Reference is current. The 350A Reference, although apparently out of production, can be serviced by Dave Belles.
The McCormack amps are older models. However, if you can pick up a DNA-1 (or DNA-2) modified by SMc Audio (Steve McCormack), it can be worked on be Steve if it ever requires servicing. He will not service McCormack amps that have not been upgraded by him. Both of these amps are large and heavy, however.
The Belles 150A Reference is current. The 350A Reference, although apparently out of production, can be serviced by Dave Belles.
The McCormack amps are older models. However, if you can pick up a DNA-1 (or DNA-2) modified by SMc Audio (Steve McCormack), it can be worked on be Steve if it ever requires servicing. He will not service McCormack amps that have not been upgraded by him. Both of these amps are large and heavy, however.

