The James Tonestack (or passive Baxandall) is a two-band tone network popular in Hi-Fi, as well as classic Ampeg and Orange amplifiers. It's a deceptively simple design, equally versatile and forgiving. It's been said that the James is incapable of a bad tone. However, those accustomed to Fender Treble/Middle/Bass controls might find it unintuitive. I encourage anyone interested to download the Duncan Tonestack Calculator, which has proven be one of the most useful tools in my shop.

For those without a Windows PC, here is a quick analysis of the Ampeg Portaflex series (B15, B25, etc.), with screenshots from the program.

Despite its simple two-knob form, the James Tonestack offers several advantages over other tone circuits. One is a nearly flat response when Bass and Treble are set at 12 o'clock:


In this plot sweeping both controls, you can see the wide adjustment range and limited interaction of Treble and Bass:


Like any passive EQ, these controls simply decrease the amount of attenuation in a given frequency range, providing an apparent boost.
Left plot is Treble knob at 75%, or 3 o'clock. Right plot is Bass knob at same setting:

treble bass

Note that this is a "shelving" style EQ, where the "Midpoint" remains fairly steady around -22dB.
In Ampeg's version, that Midpoint is about 300Hz.
Here are Treble and Bass cuts, both at about 25%, or 9 o'clock:

treble bass

Look what happens when you boost both Bass and Treble, or conversely when you cut them.
This ability to scoop or boost the midrange is another great advantage of the James.
These are gentle +/-25% settings:

midscoop midboost

Ampeg's preamp stages have a pantload of clean gain, allowing you to "recover" gain lost to the tonestack. With this in mind, a dedicated Mid control is largely superfluous. Simply shape your mids by setting Treble and Bass as shown above, and readjust your volume. Think of it as a manual transmission.

The "Range" control, featured in the Zephyr, allows you to sweep a boost or scoop throughout the entire midrange. This is similar to the "Middle" control in 70's Carlsbro amplifiers.

Left plot shows the extremes of cut and boost. Right plot shows these extremes at either end of the Range control.

cutboost range

Features in Arkham preamps such as Lo, Mid, and Hi swtiches help emphasize and focus the boosts already available in the tonestack. They are slightly different than those used by Ampeg.
Additional narrow boosts or cuts require active circuitry, as seen in Ampeg's V4 and SVT amplifiers.
While the terms James and Baxandall are often used interchangeably, the latter is actually a different, active design which places tone controls within a feedback loop.
Further reading:
Ampbooks Analysis of the James Tonestack
Duncan Amps Passive Baxandall Analysis