Sorry, I don’t really know Logic, but as you surmise, it will be pretty similar in any DAW.

Your MIDI Track is in the DAW. You need to route that MIDI Track so that triggers the MODX (MIDI Port 1). The MODX will then be able to send audio back to the DAW via USB. You can turn “Direct Monitor“ to Off (this will prevent the MODX from sending audio to its own analog outputs). I recommend this, because you will now only be sending audio to the USB Outputs — this will help you troubleshoot the configuration. When you have successfully routed and received the audio in an active Logic Pro Audio Track you will then be able to see the audio meter registering the arriving Acoustic Piano.

To turn the MODX “Direct Monitor“ to Off: [UTILITY] > “Settings” > “Audio I/O” > Direct Monitor = Off.
The Audio Signal Flow diagram will show that the internal MODX “Parts” are no longer able to feed the Analog Main Outs... the signal will be routed Out the USB port only.

On the Acoustic Piano Performance — make sure the Parts are feeding the Outputs you wish to use. You don’t mention which MODX Piano, nor how many Parts it is... but make sure all the Parts are routed to the same “Part Output”.
Select the Part by tapping the “Type/Name” box in the screen - a pop-in menu appears
Tap [EDIT] > “Part Settings” > “General” > make sure the “Part Output” is set as you desire.
If this is a multiple Part acoustic piano— set each Part to the same stereo pair.
For example, “Main L/R”

In Logic, you want to create a single stereo Audio Track set to receive audio signal from the MODX “Main L/R”... you have created an Aggregate Audio Device, so you will need to determine which of the potential Inputs to Logic Pro represent the MODX “Main L/R”.
The MODX could be as many as 10 Inputs to Logic Pro... by convention, the Main L/R will be the first two.

In order for audio to arrive and pass through Logic Pro, you must have a stereo Audio Track, armed and ready to receive the MODX Audio. Only when the Track is *active* (selected) will audio arrive and move the meters. Moving the meters of an armed/record ready Audio Track, means you are capable of recording. Monitoring it (hearing what your doing) is the next and final step.

Ensure the Outputs of that Audio Track are routed to the device acting as your principal audio output (ie, the one connected physically to your speakers by analog cables).

If you are using the MODX as you principal Audio Interface it will be able to send Audio to your speakers through the front panel “USB Audio“ knob.
If your other device is acting as the principal audio interface, then you can choose to monitor through the DAW by a setting on the Audio Track or if you wish to monitor direct, Mute the Audio Track’s signal, activate the MODX Direct Monitor, and listen through your other device.

Thank you BM. This worked. 🙂 My one concern is that the volume in the conversion is relatively low. I changed the Master MODX volume along with the USB out volume and didn't impact the recording levels. When feeding the MODX into logic the incoming MODX signal looks healthy, around 0db but the resulting wave file looks looks much more compacted (not sure that's the proper terminology) I'd rather boost it at the source than to apply gains on the resulting wave. Any help is appreciated. Thanks again!

Volume is always analog and you hear it from your speakers. While it is related to what gets recorded, it is very different. We refer to signal that is recorded/measured as LEVEL or we use the term GAIN.

So meters measure Level/Gain. Believe them when it comes to recorded signal. Record Level is totally scientific - and is set with Meters.
Volume is what you send to your speakers. It is totally subjective - and is set to taste. It has NO INFLUENCE ON RECORD LEVEL!

The pictures of waveforms you see in today’s DAW are ‘eye candy‘... again not really a measurement of anything... They usually can be sized to “look nice”... you can even color them. And while they do, generally, follow what you hear, they are typically a measurement that has little to do with anything important.

When feeding the MODX into logic the incoming MODX signal looks healthy, around 0db

excellent!

but the resulting wave file looks looks much more compacted (not sure that's the proper terminology)

I’m going to guess it is similar to most DAW where you can increase the eye-candy so it looks good! But it is not a critical setting.

Volume is how loud.
Level/Gain is an accurate measurement of the actual work the signal can do.

You can record digital audio via USB with the Volume of the MODX all the way down. Try it—setup to record audio to your DAW... In the middle of recording lower the MODX MAIN Volume to minimum. It has no affect on your recording. It is only what is send to your speakers.
Turn your speakers completely off, again Volume is analog, and has no affect on the USB (digital) signal being recorded.

Related? Yes. But huge difference. If you turn down the slider feeding the digital out, both the level and the volume are affected. However, turning down the volume has no affect on the recording.

Haha. Perfect. Ok, Thanks for your feedback BM. Much appreciated.