McIntosh MQ101 does it work as a tone control?

That's an interesting device. But I believe that Matt is saying due to the design and sonic signature of a preamp, the component itself acts as the tone control for the system with nothing in it's path to introduce noise. Plus, tube rolling adds to this capability.
At least that's what I believe and I don't look for tone controls on a preamp. However, in the digital domain, you can EQ transparently.

Does the MQ101 Environmental Equalizer, when added to any system, works like a tone control?

Sorry Matt, the answer is yes. Take a look at this article for Tone control circuit. Equalization is done by adjusting tone controls. The MQ101 has 3 tone controls, low, mid, high, and their specifications are listed here. The frequency used for low is lower than that of a typical bass control, and the frequency used for high is higher than that of a typical treble control, but they are still tone controls. Even a 10 band EQ is just a device with 10 tone controls at those specific frequencies.

O my bad, its a great EQ! (NOT) My point is, it's not a good fix. IMO you would do way better by adding a quality preamp to the system, then adding a unit that colors the sound + more cables.