This thread isn't supposed to be used for discussions, but...
Please kindly add 'Volume Leveling' on/off toggle button to the toolbar options.
The actual permanent solution to this is simple. Volume Leveling just changes the volume. It doesn't do anything else, such as Dynamic Range Compression. So all you need to do is use the EQ in DSP as AlanDistro suggests, but on the Speaker Zone, not the Headphone Zone, and reduce the speaker volume by the amount required so that the headphones and speakers match. You need to do this reduction, rather than increasing the volume against the Headphone EQ, so that you don't overpower the headphones or cause clipping. As RD James often says, always use the EQ to reduce volume, not increase it.
Once that is done, just increase the volume control you use (Internal, System, volume knob on your amp) to adjust the volume to the listening level desired. Of course, you need to be using Zones, but if you are switching from Headphones to speakers you should be using Zones anyway.
You do the above once, then never have to think about it again. No new feature required. No reason to turn Volume Leveling on and off when changing from the speakers to headphones or vice versa.
If this requires further discussion, please start a new thread.