Upright Bass Mic'd isn't a preset - it looks like something from Dave Polich (DCP sound library) so I can't really dig into this Performance to help.

The PDF describing the Performance doesn't describe about the control properties (velocities, "AF" buttons, etc) - so that doesn't help.

I would look at the Performance and look at what elements are triggered by what. Although effects (insertion, I would say for this description) can induce an octave shift - that's not very likely. I'd look for SW1/SW2 (XA Control) and velocity limits for the elements.

I wouldn't focus on Cubase vs. anything else. I'd just deconstruct the Performance to see what the possibilities are.

There's a preset where SW1 will cause harmonics to sound (which represent an octave jump). That's a possibility. Although - they could also be harmonics that are brought in by high velocities.

I'm not sitting in front of it - so I can't really help directly.

Looks like the Axxe library includes some of the basses from the Pulse library (which are basses) - and the Axxe documentation mentions a little more about controls. "Knob 1" for midrange tone control and "Knob 2" for bass tone control. No mention about harmonics or anything - not that they aren't there.

Some of the basses from the Pulse library are setup for two hands to allow for faster runs (alternating hands for runs). "Upright Bass Mic'd" is NOT (as released by DCP) setup like this. But if you have this as a re-package, maybe that's a reason. Because the scale would "fold over" (like an organ) from left-hand to right-hand. But I would doubt this considering DCP documentation mentions "Upright Bass Mic'd" is an exception to this construction.

Wow, thanks. I bought Dave Polich’s Axxe Library so long ago that I had forgotten all about it.

Here’s what the documentation says:

H16 Upright Bass Mic’d
Upright bass 1 recorded through Neumann U87 microphone, pointed towards the bass soundholes, about 3.5 feet from the instrument. Knob 1 – Midrange tone control. Knob 2 – bass tone control.

At least now I know where to look...

I would focus on your MIDI routing. Here’s why...

When a MIDI loop occurs, you are hearing the MONTAGE keyboard triggering the MONTAGE tone generator (direct) as usual, and if you have a MIDI Track active in your DAW (Cubase) that is echoing the data back to the MONTAGE tone generator, you get a doubling of notes. This is a very close timing delay, very close. And is not always recognized - but certain instrument sounds give it away...

When we do in depth classes, one of the things we point out is what this ‘doubling’ sounds like... so that people get used to what to look for...and by that I mean, what to listen for...
With drums it is usually easiest to hear, because cymbals take on a flanging quality... acoustic pianos lose their depth and become “plinky”... basses, acoustic basses, in particular, will have what sounds like intermittent phase cancellation, and you will clearly hear the fundamental momentarily disappear at some point (which will sound exactly like it momentarily jumped one octave).

When this is occurring check to make sure Local Control = Off while in contact with an *active* MIDI Track

That was it. Thanks for reminding me about Local Control.

Between the Montage and Cubase, this is a very complicated world.

... pressing the "MIDI Record on DAW" template - which I believe is what you were doing - would take care of this (Local Control) for you.
LOCAL CONTROL = Off
ARP MIDI OUT = Off
DIRECT MONITOR = On
OUTPUT SELECT = Main L/R

... pressing the "ARP Record on DAW" template would also set Local Control to OFF - but also sets ARP MIDI Out to ON - which may not be what you would want

FACTORY DEFAULTS FOR QUICK SETUP #2: ARP RECORD ON DAW
MIDI I/O = USB
LOCAL CONTROL = Off
ARP MIDI OUT = On
MIDI SYNC = MIDI
CLOCK OUT = On
REC/TRANS SEQ CONTROL = On/On
CONTROLLER RESET = Reset
GLOBAL ASSIGN (FS/SK/SCENE) = Arp Sw, 95, 92

I don’t usually use those presets. I pride myself (sometimes more than I deserve) on knowing enough to make individual settings as I need them.

In this particular case, I had been spending a lot of time learning to use the Montage Pattern Sequencer. To do this I was using the Montage as a standalone device, so I needed to have Local Control turned on.

Then I started using Cubase with the Patterns I had created, and totally forgot about turning Local Control off.

I think there’s a good chance that I will recognize this problem sooner next time.