Source: man snd_hda
To enable bass/treble controls for pcm0, add the following to /boot/loader.conf:
hint.pcm.0.eq="1"
For me the front headphone jack was plumbed to pcm1. To dump all information about the current configuration, do:
sysctl dev.hdac.0.pindump=1
dmesg
hdaa0: nid 0x as seq device conn jack loc color misc
...
hdaa0: 20 01014410 1 0 Line-out Jack 1/8 Rear Green 4
hdaa0: Caps: IN OUT HP Sense: 0x80000000 (connected)
...
hdaa0: 27 02214c1f 2 0 Headphones Jack 1/8 Front Green 12
hdaa0: Caps: IN OUT HP VREF Sense: 0x00000000 (disconnected)
In my case the rear audio jack has node ID (nid) 20 and is set to association (as) 1 and sequence (seq) 0. As for the front headphone jack nid=27, as=2, and seq=0. Changing it to as=1 will put it in the same group as the rear jack and setting seq=15 will make sure the rear jack gets muted when using the front jack. Add to /boot/loader.conf:
hint.hdac.0.cad0.nid27.config="as=1 seq=15"