After reading some schematic diagram, I think getting head unit with Sub out is not easy solution. The Amplifier itself also received ANC signal when producing audio signal to the subwoofer, either it is clearing the ANC ambient noice or adding ANC ambient noise. My guess it is clearing it. Here are what I found from Honda tech:
The stereo amplifier is a separated power amplifier interface. Digital audio signal (SPDIF2) that is received from the audio unit is
processed on the DSP, and sent to each speaker after amplified by the AMP IC.
The stereo amplifier communicates with the audio-navigation unit or the audio unit using the dedicated serial bus (RS485). This
communication includes the vehicle speed (for SVC) information, audio settings, and the MUTE commands. The stereo amplifier
adjusts the speaker outputs by oneself which is based on the received information.
The stereo amplifier receives the noise cancelling signal (ANC), guidance signal, and system beep signal via independent lines, and
mixes them to the applicable audio lines.
Active Noise Cancellation (ANC)/Active Sound Control
Both the active noise cancellation (ANC) and the active sound control are built into the ANC/active sound control unit. Both systems
eliminate unwanted sound entering the passenger compartment. There are two microphones mounted in the headliner; one is just
behind the front overhead roof console and the other is just ahead of the rear map lights. The microphones capture low frequencies
entering the passenger compartment and send a signal to the ANC/active sound control unit. The ANC/active sound control unit then
creates the sound wave that is sent out through the speakers to eliminate the unwanted sound.
Active Noise Cancellation (ANC)
The ANC system works to cancel the engine booming sound up to about 2,000 rpm (min-1). The ANC/active sound control unit
receives the engine speed pulse (NEP) from the PCM and outputs a sine-wave-sound through the audio speakers to cancel the lowfrequency
noise from the engine. The ANC system also uses two microphones to detect and monitor the low frequency noise in the
passenger compartment. The microphones feedback the noise information to the audio unit or the audio-navigation unit which adjusts
the speaker output to reduce the noise. The ANC also receives the output of the door open/close signal from the MICU. Whenever
any door is opened, the ANC system temporarily stops working. Each signal is sent to the ANC/active sound control unit via F-CAN.
The ANC/Active sound control unit monitors the stereo amplifier output signals to the subwoofer, and if the subwoofer gives a sound
more than the rated value, the ANC/Active sound control unit stops ANC control. This control is not for deleting bass of an audio
About the HD Radio, I think it is still possible to swap the tuner unit
The tuner unit is a separate digital radio tuner interface. Received AM/FM radio waves are processed on the digital signal processor
(DSP) and sent to the audio unit via the dedicated digital audio line (SPDIF1). At this time, HD radio(TM) sound signal is separated
from normal AM radio sound signal. All of the information about AM/FM reception (includes RBDS information) are sent from the tuner
unit via the dedicated serial bus (RS485). This unit holds AM/FM radio frequency presets even when the 12 volt battery is
For vehicles with Sirius XM® radio, the antenna for receiving AM/FM radio waves and satellite/terrestrial (XM) signal uses a roof
antenna type. AM/FM radio wave is amplified by the AM/FM antenna amplifier. The sensitivity of the FM wave is raised by the auto
gain control. The XM antenna receives satellite signal and terrestrial signal by using the high-gain antenna. Signals are sent to the
audio unit via the dedicated single line (SAT/TER).