There must be some whacky on-MoDX setting I'm missing here, but can't fathom it.

Usually the missing item is missing from the description. How are you connected to the MODX? And what MIDI I/O Mode are you using?

Press [UTILITY] > “Settings” > “Advanced” >
“MIDI I/O Mode” should be set to “Multi” — this will allow the MODX to transmit and receive MIDI on multiple channels (that is what you want)... each Part can receive cc7 for Channel Volume.

You cannot use “Single” - here all the MODX Performance KBD CTRL Parts will be addressed on a single MIDI channel; quite naturally individual Part Volumes via CC are not available. All non-KBD CTRL Parts are Off (and will not transmit or receive)

You cannot fully use “Hybrid” - here all the MODX Performance KBD CTRL Parts will be addressed on a single MIDI channel, quite naturally individual Part Volumes via CC are not available for the linked Parts. All non-KBD CTRL Parts will transmit/receive on the correspondingly numbered Channel and therefore will respond to cc7 as you expect.

This explains the behavior you are seeing.
The MODX Parts will receive cc7 (as you expect) when MIDI I/O Mode = Multi.
In Hybrid all non-linked Parts behave like the setting was Multi, while all KBD CTRL linked Parts behave like Single (thus the name “Hybrid”... it combines the two behaviors).

Hope that helps.

Thanks for the reply, I wasn't clear in my original post - the nano is plugged direct into the MoDX via the USB device port.

I've tried all three midi I/O modes, and in multi the behaviour is the same as above - CC 07 affects all channels/parts regardless of channel it's sent on. Also, as I said, if I select the GM/GS "Multi/GM" performance, the sliders behave as desired. Switching to a standard performance, and each sliders affects all >_<

It is most vexing.

One thing you'll notice about Multi/GM is that PARTs 2-8 all have keyboard control turned off.

With the USB TO DEVICE port - connected devices will act as if all of the Keyboard Control = ON Parts are merged into a single channel.

Whenever channel merging is going on (as with single and hybrid mode) - this takes away the ability to individually address the CCs of those merged parts (they're acting as one).

In the case of USB TO DEVICE - the connected USB device is addressing all of the Keyboard Control = ON parts as one (even in multi-channel mode).

If you want the Nano to individually address the sliders - you'll need to turn off keyboard control for those Parts. This may not be what you want in terms of local keyboard control because you may want those parts to all be under local keyboard control. This is where you'll have to find another way as one need is incompatible with the other.

I've tried all three midi I/O modes, and in multi the behaviour is the same as above - CC 07 affects all channels/parts regardless of channel it's sent on. Also, as I said, if I select the GM/GS "Multi/GM" performance, the sliders behave as desired. Switching to a standard performance, and each sliders affects all >_<

It is most vexing.

Connect to the MODX using one of the two MIDI port options: the 5-pin MIDI In or from a computer/tablet/ via the USB-MIDI “To Host” (not the “To Device” Port — for how the MODX behaves when connected to the “To Device” port, see ‘Extra Credit’ section below).

Each slider affects all because they are all treated as if they are on the same channel due to your connection... all KBD CTRL Parts will respond to your Controller’s single MIDI Channel In. Sending any standard CC will change all Parts as if they were on the same channel.

When you use the “To Device” port - this allows a controller capable of only Transmitting Out on one channel to take the place of the MODX keys. The MODX speaks to itself directly, when in Multi it generates MIDI commands for all linked Parts. This means your external controller will behave the same as playing the MODX keys - playing “Pearly Gates” a Perf with 3 KBD CTRL linked Parts, can all sound together, even though your controller only sends on one channel.

Extra Credit: How the “To Device” MIDI Connection works
__ When MIDI I/O Mode is set to Multi
• Channel events, received through channels corresponding to Parts with Keyboard Control Switch set to On, are received in parallel by all Parts with Keyboard Control Switch set to On. After receiving, when this instrument transmits MIDI data, all note data of the channel corresponding to parts with Keyboard Control Switch set to On are transmitted.
• Channel events received via the channel that corresponds to the Part with Keyboard Control Switch set to Off affect the corresponding Part. After receiving, when this instrument transmits MIDI data, the same channels used for receiving are used to transmit the channel

__ When the MIDI I/O Mode is set to Single or Hybrid
• Channel events, received through channels corresponding to Parts with Keyboard Control Switch set to On, are received in parallel by all Parts with Keyboard Control Switch set to On. However, when this instrument receives Control Change messages regarded as Common parameters and the MIDI I/O mode is set to Single, this instrument does not send Control Change messages to the Part and instead treats them as Common parameters. After receiving, Sending of Control Change messages is done according to the MIDI I/O Channel setting.
• Channel events received via the channel that corresponds to the Part with Keyboard Control Switch set to Off are received by the corresponding Parts. The received MIDI data is not re-transmitted when the MIDI I/O mode is set to Single.