There are no reported problems with Firmware 1.60 (2011). That was the final firmware update on the XS series, released one year after it was discontinued.

What you describe does not sound like an issue with the firmware version. I don’t believe there would be any benefit in downgrading your firmware...
If you care to describe exactly what you are trying to combine, we maybe able to find the cause and offer a solution.

Possible causes:
Delayed notes could be programmed to be delayed (envelopes, Effects etc); It could be caused by polyphony overload (exceeding 128 note limit); It could be caused by a MIDI routing issue...
Is a Sustain pedal involved?
Do any of the Preset sounds exhibit this same behavior?

Allright! Thanks for your response.

For example:
I made a performance with 4 instruments
1. Nu WarmPad 2 elements
2. Full Concert Grand 8 elements but connected to different velocity's/ note ranges
3. Soft Case 7 elements but connected to different velocity's/ note ranges
4. Dee Tune 1 element

I use this performance to play different parts of songs after each other without switching sounds, so without cut in the sound.
Let's say that I play 6 notes at a time, this will result in 6 x 2 (warmpad) x 6 (Full concert) x 5 (soft case) x 1 (dee tune) = 360 notes
With some sustain pedal usage even more.
The maximum poliphony of the unit is 124 notes, so I do think I made some calculation mistakes...

Thanks in advance!

“Nu WarmPad” uses 2 notes of polyphony per Key played
“Full Concert Grand” uses 2 notes of polyphony (as it is a Stereo Waveform)
“Soft Case” uses 1 note of polyphony
“Dee Tune” uses 2 notes of polyphony per Key played.

Elements only use polyphony when they actually are generating sound. Please see your Motif XS Data List (booklet 1) for a handy Voice List which indicates how many simultaneous Elements are used when playing each Voice.

Playing a six note chord uses exactly 42 of the 128 note capacity... assuming that each of these sounds play across the entire keyboard.
7 Elements are sounding for each Key pressed...
Therefore, the programming is fine, your issue is the Sustain pedal. This multiplies the polyphony usage tremendously.

Be sure to disengage the Sustain Pedal for Instrument Parts that don’t have one in the real world (this can help).

For example, the Bass sound likely (not always, but often) can do without Sustain. In your Controller setup deactivate Sustain for Part 4.
The pad sound can also probably do without the Sustain pedal... the pad typically is used behind the acoustic or electric piano... try deactivating the Sustain function for Part 1, as well.

These are just suggestions, you may find the piano and electric piano responding to the Sustain pedal accomplishes what you require, and if your pad sound only sounds when you’re actually holding down keys, this will remove the blur of sound on piano flourishes. The pad will function when you hit and hold (filling in the background) but will be out of the way on (clearly) piano-type arpeggiated flourishes.

You can set the Receive Switches for each Part, this will determine if the Part responds to the Controller, or not.
Press [EDIT]
Select the PART using 1-4
Press [F6] RCV SWITCH
Uncheck Sustain for Parts that don’t have Sustain pedals.

If you are moving between sounds using the front panel Control Sliders, you might consider using the Mute buttons instead.
Here’s how and why:
After recalling your Performance, press the [PERFORMANCE CONTROL] button
Now reading the legend under the numbered buttons use buttons [9]-[12] to Mute Parts 1, 2, 3, and 4, respectively, when you don’t need them..
When they are Muted they do not request polyphony. Lowering the Control Slider prevents you from hearing the polyphony each Part is using, Muting stops the Part from requesting polyphony, completely.

Hope that helps.

Wow! Thanks for this detailed response!
I will use this tips and improve my layered sounds!
Thanks really a lot!