I found a guide that explains step by step how to compile a version of Darkice that will support AAC.
https://github.com/Laav/darkice14-libaacplus-rpi-guide
Sadly, something in that process now stops ST from running. So I now have a setup which simply broadcasts the audio as received without any of the lovely ST processing.
I noticed that aplay -l shows that the HifiBerry card has moved from card 0 to 1 (swapped with the alsa loopback) and I updated asound.conf but it has made no difference.
pi@raspberrypi:/etc $ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Loopback [Loopback], device 0: Loopback PCM [Loopback PCM]
Subdevices: 8/8
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
Subdevice #3: subdevice #3
Subdevice #4: subdevice #4
Subdevice #5: subdevice #5
Subdevice #6: subdevice #6
Subdevice #7: subdevice #7
card 0: Loopback [Loopback], device 1: Loopback PCM [Loopback PCM]
Subdevices: 8/8
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
Subdevice #3: subdevice #3
Subdevice #4: subdevice #4
Subdevice #5: subdevice #5
Subdevice #6: subdevice #6
Subdevice #7: subdevice #7
card 1: sndrpihifiberry [snd_rpi_hifiberry_dacplusadcpro], device 0: HiFiBerry DAC+ADC Pro HiFi multicodec-0 [HiFiBerry DAC+ADC Pro HiFi multicodec-0]
Subdevices: 1/1
Subdevice #0: subdevice #0
pi@raspberrypi:/etc $
Something is making ST unhappy!
pi@raspberrypi:~/Downloads $ ./stereo_tool_gui_pi2
Thimeo Stereo Tool 9.90 (for Linux) (C) Thimeo Audio Technology, 2001-2022
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.front.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM front
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround51.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround21
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround51.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround21
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround40.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround40
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround51.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround41
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround51.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround50
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround51.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround51
ALSA lib confmisc.c:1281:(snd_func_refer) Unable to find definition 'cards.HifiberryDacpAd.pcm.surround71.0:CARD=1'
ALSA lib conf.c:4568:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5047:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM surround71
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.iec958
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.iec958
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.iec958
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.hdmi
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.hdmi
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.modem
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.modem
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.phoneline
ALSA lib pcm.c:2565:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.phoneline
Segmentation fault
So back to the drawing board as they say!