Hi Ian, Welcome to YamahaSynth!

I have been using a stored pattern with voices stored in the 16 tracks so I can access my required voice at a touch of a button.

This is the key to what you need to do.

For example, I like the 'S700 for XS' and 'Lush' voice as a combination, can this be saved as a performance, and can I reference this performance at a touch of a button?

When in PATTERN MIXING, you can make combinations by manipulating the MIDI RECEIVE Channels.

By default Part 1 is set to receive on MIDI Channel, Part 2 on Channel 2, and so on through Part 16 on Channel 16.

You can layer the "S700 for XS" with the "Lush" strings by simply placing them on the same MIDI Channel. Say the "S700 for XS" is in PART 5 and the "Lush" strings Voice is in PART 6.
Press [MIXING]
Press [EDIT]
Press [5] to view Part 5 parameters
Press [F1] VOICE
Press [SF2] MODE
Here is where you can see Part 5 is set to "Receive Channel" 5

Press [6] to view the "Receive Channel" for Part 6
Set it also to Channel 5 (you will notice immediately the strings no longer sound)

By placing both on the same receive channel you will be able to have them sound at the same time: whenever you press button [5]

Press [STORE] to store your Pattern and Mixing setup.

Now press [5] to hear both piano and strings.
Buttons [1]-[4] will sound the individual sounds assigned to those Parts, button [5] will now sound the piano and the strings. Button [6] will be silent because currently nothing is assigned to MIDI Receive Channel 6...

You can assign as many Parts as you desire to the same MIDI Channel.
You have the LIMIT parameters to setup Note Limit regions for splits. [F1] VOICE > [SF3] LIMIT
You have [F2] OUTPUT to set the relative volumes.
Each Part still has a full compliment of editing parameters, including the "Receive Switch" ([F5] RCV SW) which instructs the Part to follow or not a particular Controller. For example, you might want to have the piano ignore the Expression pedal, while the strings follow the pedal; you might want the piano to follow the sustain pedal while the strings ignore the sustain pedal, and so forth.

Hope that helps.

Thanks that did the trick.

Bad Mister, is there any way once you've assigned multiple parts in a mix to the same channel to hear the sound(s) while edited. Whenever I edit a parameter on a the track the midi channel changes to that track which makes the sound go silent (because no sound is assigned to that midi channel). So if I've got 3 voices layered in a mix on tracks 1-3 all assigned to channel one, when I try to balance volume or filter offset on voices 2 and 3 I can't hear anything (because track 2 or 3 gets selected which transmits to midi channel 2 or 3).
I just purchased a MoXF 8 - it's quite amazing. I'm coming from a Kurzeil PC3X - a fine synth - but it weighs too much for me to keep lugging around at my age (taking it's toll on my back). I'm looking forward to being "creative" with music again - the MoXF gives me lots of ideas and capability - where the PC3X is more just a performance keyboard rather than a workstation. Thanks for the help!

Bad Mister, is there any way once you've assigned multiple parts in a mix to the same channel to hear the sound(s) while edited. Whenever I edit a parameter on a the track the midi channel changes to that track which makes the sound go silent (because no sound is assigned to that midi channel). So if I've got 3 voices layered in a mix on tracks 1-3 all assigned to channel one, when I try to balance volume or filter offset on voices 2 and 3 I can't hear anything (because track 2 or 3 gets selected which transmits to midi channel 2 or 3).

yes there is! Hi, William... It's a matter of transmit and receive.

Well, if we are transmitting to the Tone Generator via the MIDI channel of the currently selected track, yes, quite right you cannot select [2] or [3] and hear anything because no internal sounds are assigned to those MIDI channels. You have assigned them to respond only when you are transmitting on Channel 1.

So the solution is to set TRACKs 2 and 3 to transmit on MIDI CHANNEL 1.

Let's use the items you mentioned in your post... To adjust relative Volumes you will want to play all three PARTs together (simultaneously).
Press [SONG] or [PATTERN] whichever you are using for these three Parts
From the main mode screen:
Press [F3] TRACK
Press [SF1] CHANNEL
Set the Transmit Channel of Tracks 2 and 3 to Channel 1

Now when you select those Parts to Edit they will sound.
Now when you press [MIXING], select a PART 1, 2 or 3, you can use the [F5] VCE ED (Voice Edit) to drop into full Voice edit, you can access all the parameters you may want and hear what you are editing.

Any of the 16 numbered TRACK buttons can be set so that you can transmit OUT on any channel... It defaults so 1-16 are assigned to channels 1-16 respectively (quite naturally) but you can change it as you require, whenever you require it.

Hope that helps.

Thanks, that was a great help - that was exactly my issue, the song tracks were transmitting on their default channels. I'm sure there will be more to come but I will read the manuals and experiment before asking.