Your question is about the the built-in audio interfaces in the Yamaha Synths and Stage Keyboards. Rest assured, they all output studio quality audio.

Perhaps what you are confused about is their role as audio interfaces for a Windows or Macintosh computers.
AN AUDIO INTERFACE can do the heavy-lifting when attempting to get audio into and back out of your computer... converting analog signal to digital, and digital signal, from the computer, back to analog so it can feed your speakers. Typically the specs are listed by the number of INPUTS they allow the computer to ‘see’ simultaneously. Therefore, the Outputs from the keyboard are seen as Inputs to the computer. The number of OUTPUTS listed on the specs typically, is the number of audio bus that are returned from the computer back to the audio interface. Example: the MONTAGE can send 32 bus Outs which are seen by your computer as 32 digital Ins.

A “bus” is a vehicle for transporting passengers (in this case signals from the synth) to a set destination, along a specific path or route. Multiple passengers can ride the bus. Or you can isolate signals on their own discrete bus if you desire. By default, all 16 MONTAGE synth Parts plus the AD IN Part are ‘riding’ the Main L/R buses... You can reassign them, as necessary.

MONTAGE 6/7/8
The MONTAGE can function as a 32-in/6-out, 24-bit audio interface for your computer when you operate at 44.1kHz. The AD IN can utilize the A-to-D capability of the MONTAGE. What that means is there are 32 simultaneous Outputs from the MONTAGE... You can configure these as individual buses or configure them as odd/even stereo pairs, as you require.
16 Stereo/44.1kHz: Audio data sending capability for the instrument is a maximum 32 channels (16 stereo channels) at a sampling frequency of 44.1 kHz.
4 Stereo/44.1-192kHz: Audio data sending capability for the instrument is a maximum 8 channels (4 stereo channels) for a sampling frequency of 44.1 kHz to 192 kHz.
NOTE When “4 Stereo/44.1-192kHz” is selected, available frequencies are only 44.1 kHz, 48 kHz, 96 kHz, or 192 kHz.

MODX 6/7/8
The MODX can function as a 10-in/4-out audio interface for your computer. The AD IN can utilize the A-to-D capability of the MODX.
The MODX operates at 44.1kHz, 24-bit

Stage Keyboards: CP88/CP73 and YC61
The Stage Keyboards can function as their own stereo audio interface @ 44.1kHz (2-in/2-out), 24-bit audio. The Stage Keyboards do not have an AD IN... the External Input jacks on these products allow you to send the external device’s audio to the analog outputs of the Stage Keyboard (ie, no conversion to digital takes place, no signal is sent from the External Inputs to the computer as in the MONTAGE/MODX synthesizers. The external analog signal is simply routed to the analog outputs of the CP/YC.

MX49/MX61/MX88
The MX can function as their own audio interface @ 44.1kHz (2-in/2-out). The MX do not have an AD IN... the Aux Input on these products allow you send the external device’s audio to the analog outputs of the MX (ie, no conversion to digital takes place, no signal is sent from the Aux In to the computer as in the MONTAGE/MODX synthesizers. The Aux signal is simply routed to the analog outputs of the MX.

Extra Credit:
In general, there are concrete reasons for the pricing of instruments — it is a common belief that these things are arbitrarily priced — nothing could be farther from the truth. The MONTAGE allows those who perform live but also record audio, the ultimate in flexibility when it comes to routing for recording.

Try to imagine the after market research and then all the subsequent (international) meetings that go into deciding which set of features go into which products. No company can make enough models to please every customers situation. But the number of Outputs/Inputs and the sample rate of operation even on the least expensive item are all professional studio quality.

The top-of-the-line MONTAGE should have the best option (I’m sure I can get you to agree on this fact).
Given the ‘title’ Stage Keyboard, points to its main design function. Keeping it lightweight, powerful and most of all, sounding great are design goals (and reasonably affordable). The fact that you do not have to buy an additional device in order to record to a computer (other than a USB cable) is an advantage.
If you want to record say at 96k or 192k you should invest in a 2-in/2-out audio interface, like the Steinberg UR22C

Extra Credit: Audio Interfaces
Steinberg Audio Interfaces

Hey Bad Mister,

I have a 192k interface. So if I was to take a CP88 and hook the audio outputs to the 192k interface I would get a true 192k wav file on my Daw? I'm using the MODX 6 now and I just go audio over usb for the 44.1k. I want to get a 88 digital/ stage piano to go with my MODX 6 with the best audio quality I can get. I know the Montage would be awesome, but it's a little more than I want to go. So would the CP88 be about the same audio quality as my MODX 6?

If I would have known when I got my MODX 6 that I was going to want an 88 keyboard I would have bought the MODX 8, but to late now.

Thank you sir,

Bill

Yamaha can answer this better. I don't really know what resolution the digital signal at the stage just prior to the CP88's DAC (digital-to-analog converter). But lets say internally the highest resolution of audio for the CP88 is 44.1kHz/24-bit. Yes, it's better to oversample the analog output. The DAC may be doing some interpolation to "smooth" the output and sampling this at a higher frequency with your audio interface would be "better". But this isn't the same as starting with a higher resolution signal to start with.

As BM mentioned - all of these are studio grade. "CD Quality" is 16-bits, 44.1kHz.

See: https://www.soundguys.com/high-bitrate-audio-is-overkill-cd-quality-is-still-great-16518/

Here's a summary of the above article:

44.1kHz is (for most people) twice the sampling rate of the highest frequency for human hearing. This means frequencies in the range of human hearing can be represented using 44.1kHz.

The extra bits between 16-bits and 24-bits (which the CP88 has) is useful in the studio for mixing. To have higher dynamic range. However, for listening - the difference is not useful as the extra bits do not provide an audible difference unless you're listening at the output with earphones and crank the volume up to unsafe levels.