Increasing vent length lowers F3, Fb, so for all but VLF special effects and some pipe organ symphonies, tuning lower would normally only be called for if the bass was too 'boomy' which can occur with corner loading. For more bass/mid-bass 'presence' [tonal balance] then, either a larger vent that raises F3, Fb and/or some baffle step compensation [BSC] is called for which is usually the case if the speakers are well away from any wall, corner boundaries. Of course, fiddling with these or any other fine tuning adjustments should only be done once installed in-room.

As for tuning by ear, it's better than nothing, but due to the way we perceive sound it's a crapshoot unless one has perfect pitch good enough to tune a piano and why I got in the habit of getting young women to help since men tend to do better down low while women do much better through the HF, so between the two, a decent balance can normally be struck and I'm guessing why I didn't have a clue what all SAF, WAF, etc. meant/implied when I got on-line.

GM