The premise is that the ultimate modular mixer can also invert and/or offset the signal, whether it be processing audio or control voltages. In the audio case, an inverter allows you to create new sounds by mixing together more than waveform from an oscillator or more than one output from a multimode filter, and inverting one of the inputs to create interesting harmonic cancellations and reinforcements. Adding an offset to audio waveform allows you to create asymmetrical distortion when fed into a wave shaper, or overdrive a filter or ordinary mixer. (For more on this and utility mixers in general, see http://learningmodular.com/choosing-utility-mixers-for-your-modular/)
The top row contains mixers that can also invert and offset the signal in a pretty straightforward manner.
The middle row contains more complex control voltage processors which can achieve the goal of mix, invert, and offset, but they may require a bit more thought or patching than the top row. Although less intuitive or immediate, the upside is that these modules should provide even more creative possibilities in your modular.
The modules in the lower row are missing one of our desired features, but are still very interesting mixer alternatives in their own right. For example, the RYO Ampmix can provide up to a 2x boost for each signal it mixes, allowing you to overdrive modules that don't have that option built into their own input controls.
If you know any other suitable modules on ModularGrid that I missed, let me know and I'll add them to this rack.