Salt gives you 6x DC-coupled outputs -5V to 5V, 2 of these are the audio outputs (at 44.1kHz) and 4 of these are the analog outputs. Salt+ adds 4 more analog outs at -5V to 5V. You have the usual restrictions in terms of analog channel count and sampling rate: 4 channels at 44.1kHz or 8 channels at 22.05kHz.
Notice that Salt requires a dual ±12V power supply (standard Eurorack PSU).
The audio expander capelet goes on top of the Bela cape and does not increase the channel count, it simply provides AC-coupling and basic filtering for Bela's regular analog outs (usual sampling rate/channel count restrictions apply). The output swing is the same as regular analog outs, but AC coupled, so -2.5V to 2.5V.
The CTAGs have no DC-coupled outputs: the outputs are all audio AC-coupled. CTAG runs on the BeagleBone Black (or green!), so if you have one around from a past Bela, you can use it. You can stack a Bela cape on top of it to get the analog I/O from it (not the audio though), but I don't see how this would help in your case.
jestern77 About the Pepper I understood that it can output Modular levels using a zener diode and a resistor but I'm not sure I completely understood right.
No, the Zener can be used to modify the range covered by the pots. The analog outputs are always fixed at 0-5V, and the audio out is DC-coupled 0.4V to 2.2V.
Also notice that the upper end (e.g.: 4.85V to 5V) of the output range of the analog outs on Bela and Pepper could be fairly noisy depending on the power supply in use. Salt/Salt+ gets very close to the +5V without that extra noise up there.
So to sum up:
jestern77 CASE OF 6 Channels:
4x DC coupled 0-5 Volts.
2x bipolar -5V 5V DC coupled (I can shift it with an extra offset circuit in case)
Salt seems good
jestern77 CASE OF 8 Channels:
2x normal audio
4x DC coupled 0-5 Volts.
2x bipolar -5V 5V DC coupled (I can shift 0-10 with an extra offset circuit in case)
Salt and Salt+ together will d. The audio output will come out in the -5V to 5V range, so you may have to attenuate it (e.g.: with a passive resistor divider or a pot) if you are going into some equipment that doesn't expect such a large swing.