Upon checking the manual I find that the 2.5mm balanced output is specifically intended for balanced-headphones and is not a differentially-balanced line output as we know it as used for microphones and pro-interconnects using TRS or XLR-3 connectors.
Balanced headphones use a floating ground arrangement which includes no ground-reference connection. That's 4 conductors in total and as such the 2.5mm headphone output jack is a 4-conductor TRRS jack. I would not use a TRS (3 conductor) 2.5mm plug in this jack, regardless if it is capable as acting as an unbalanced line output or not, as tying the signal (-) returns together could damage the deck.
If this was a differentially-balanced 2-channel line output it would need 5 or 6 conductors depending on if the ground connection was shared between channels or not, just like a pair of balanced microphone lines or interconnects.
The manual is not clear on this but seems to indicate that the 3.5mm jack is the one intend for line-output, and that setting the player to LINE OUT disengages volume attenuation on the playback deck when in that mode.
Below are the relevant sections copied from the manual-
(page 4) Part Names
3. [Icon] Tap when using balanced headphones and switch between the "Balanced" and "ACG" modes.
Balanced: Compared to the normal single-ended operation, this mode enables you to get a larger output and as the high voltage is not grounded together with the small signal, you can anticipate superior music quality. Because you get a larger output, you can drive headphones with a higher impedance. ACG: The basic operating method is the same as Balanced (mode), but uses technology to even more forcefully fix grounding standards, so you can enjoy a sound quality with improved clarity and stability than Balanced (mode). Output volume is the same as the regular single-ended operation, however.
4. [Icon] Line Out: You can switch the headphone output to the line output mode to connect external equipment.
(Page 19) Line Out
You can connect a headphone amplifier or D/A converter, for example, that has line input. You will need a headphone plug (commercially available) to connect to this unit. Make sure you have the correct type of headphone plug for the jack of the device you are connecting.
Connecting a device
1. Connect the device to this unit through the headphone jack.
2. Turn on the power of this unit, and select "Line Out" in the home screen to turn Line Out on. "Line Out" is displayed on the status bar on the screen when this is on. When "Line Out" is on, output through the headphone jack is at maximum volume, so do not use headphones. Adjust the volume on the connected device.
(Page 21) Line Out Mode
"On": Switch the headphone output to the line output mode to connect external equipment.
When "Line Out Mode" is on, output through the headphone jack is at maximum volume, so do not use headphones. Adjust the volume on the connected device. When turned on for European models, you can then select either "Fixed (default value)" or "Variable". When "Variable" is selected, volume can be adjusted on this unit. If you select "Fixed", adjust the volume on the connected external device.
"Off (default value)": Line output is not enabled
(page 31) General Specifications
Interfaces and input/output jacks
Headphone jack: 3-conductor, Ø3.5 mm, impedance: 16 to 300 ohm *
Headphone jack (balanced output): 4-conductor, Ø2.5 mm, impedance: 32 to 600 ohm *
* Do not connect headphones that have an impedance outside the rated range as this may result in damage.