And therein lies my concern, the Superphon SP-100 is a legacy zero gain, zero feedback, direct-coupled, high slew rate impedance converter, and input selector with a decent quality stereo pot. In addition, Stan Warren upgraded this unit a few years back to modernize it.
The 4B is an '81.
I've been using a 120 watt class D diy amp with the Superphon, and it works fine, so I can't beleive the 4B will be an issue, but you never know. (The 4B is out for repair and hasn't been hooked up yet).