I am no real expert on this when it comes to balanced signals. But I will tell what I generally do on unbalanced signals first.
I have done several volume boxes of this kind for unbalanced signals. I use them connecting the phone out from various stuff to an unbalanced line in on just about anything.
The simplest version only needs a stereo potentiometer. Generally a logarithmic 10 kOhm potentiometer is a good choice. Any value really goes as long as it is not too low, because this is the new loading the output will see. These can probably be found in just about any radio parts store all over the world. If possible get one with a plastic resistor instead of carbon as they tend to be less noisy.
In this case each channel of stereo has two connections, live and ground. Basically you can tie all the grounds (both inputs and outputs) together at one point. You may end up in a groundloop situation if there is a lot of mains signals around, so you might select to leave one of the input grounds unconnected.
Next connect the input signal to the "high" input of the potentiometer. Generally this would be the right connector when you view the pot from the front. Ground goes to the other signal, and the wiper or middle connector is the output signal.
I have not done it on a balanced signal, but I can see some possible variations. First of all, the grounds all go together as before, but check for ground loops.
a) a fourgang potentiometer. I have seen them, but they are not as easy to find as a stereo pot. Connect each of the four in the same way as for the unbalanced.
b) separate left and right volume
c) a little more elaborate thing using two resistors and one potentiometer per channel. Take the two input signals (hot and cold) and run them through a resistor, say around 2 kOhm or a bit higher. Now connect a pot between the two signals after the resistor, in effect shorting the hot and cold to each other in higher or lower degree. On the pot one end goes to the left terminal (as seen from shaft) and other to middle. Experiment a bit with different size resistors.
So good luck, heat up your soldering pen and start experimenting. Put the speakers on low volume first, and be sure to not shortcircuit the output signals. In general professional audio equipment is well protected by it does pay with a bit of caution.