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Be sure to look into MANUFACTURER SUB-FORA as well..
I use an outboard mixer and 1/8th to 1/4" cables to sum and mix a number of signals and do effects sends etc. I still have a few mixers on the synth. One of them is the Pittsburgh which I mostly use for attenuators. The others are a matrix mixer and mutamix which are probably too big for what you are looking for.
Yeah, I also have to ask what is wrong with the a138b? Im happy with most of my doepfer utility modules other than maybe the crossfade which has some scaling issues.
A Klavis Mixwitch would fit nicely in there. It’s small, relatively inexpensive, has log or linear response, can be configured as 4:1 or 2:1, provides voltage out per channel, and can be stepped clicklessly per channel in a variety of ways. It’s a super useful module.