I have managed to draw up the schematic of the relevant portion of E10 and the components to be removed in order to install the crossfeed circuit. Unfortunately, the bass boost switch is referenced to ground with the common contact. If it should be used for for the crossfeed, the ground PCB trace or the switch contact pins need to be cut and I'm not sure if I want to do that. The schematic also shows where to attach the crossfeed circuit with eight wires plus ground.
So I conclude:
1. It's possible to add a crossfeed circuit
2. It's not difficult
3. If the crossfeed circuit is to be enabled/bypassed using the bass boost switch, irreversible modifications of the original PCB or components are required.
4. A permanent crossfeed circuit can be added with only four wires plus ground and without irreversible changes. I think most people (including me) wouldn't want that.
I also found a possible cause for the bass boost imbalance that some people described. One pin of the switch wasn't properly (hand-)soldered. Although this had no effect in my amp, I can imagine that this could look worse in other amps.
In the first step I will modify the bass boost as described and then I will think about if I want to do the irreversible modifications to my amp. I will try to post frequency response measurements of the modified bass boost in the next days.