What you can do is get a modern, no-tone control pre-amp with a processor pass through input, send an output from that pre-amp to an older pre-amp with tone controls, return the output from that pre-amp (with the signal "tone-controlled") back to the modern pre-amp (the processor input). You can then switch the tone controls in and out. For instance, a recent Audio Research line stage and the old SP-3 (the last ARC I remember having tone controls).
