this seems to be a common problem with modern 12string guitars. I've encountered the same calamities since I've bought my Taylor 555 a million years ago.
I just wanted to try an open D (minor/major) tuning and found the lower strings not slippery but slack and unresponding since there wasn't enough string tension because of the downtuning (2 halftones from regular tuning).
Logical consequence (I thought): Tried heavier strings. This means a .011 set instead of standard 0.010s.............Ouch! Big Mistake!!!! While tuning up to regular pitch the Top was bellying heavily due to higher string tension. I didn't even try open tuning.....I just dumped the strings.
In short: Modern guitars have very thin tops that are adjusted to regular tuning with 0.010 string sets. This Means you have to compensate the string tension when tuning down.
4 semitones on the low C-string
2 semitones on the low A-string
2 semitones on the d-string
I am still searching for the right string gauges to compensate for the downtuning.
It ist NOT a question of adjusting the truss rod i.e. the neck of the guitar since the top/soundboard is considerable more sensitive to changes in string tension than the guitar neck. The tweaks on the guitar neck (back bow/ front bow â†’ convex/concave) is a more long-term adjustment when you've finally chosen YOUR string set-setting)
After a few days on openC tuning with regular strings the top has been sinking in that the low C strings produced a considerable amount of buzz and capoing on the 6th fret did not deliver a clear tone (much buzzing).
AND: 12-string guitars have a very low string action compared to 6-string acoustics (just like an electric 6-string) to accomodate easy fingering (bar-chords!!!! Ouch!!!!!!!).
On the D'Addario Web site (as far as I remember) you may find a table with string tensions.........maybe this could help (Think I'll give it a try and maybe a bit maths.....).
Electrifying the guitar might also help. You don't have to dig in that much to get a really loud tone ;)
I think asking John directly would be the most straightforward approach to solve this mistery.........does anybody know his mobile number?
EDIT: Yes, there ist is: http://www.daddario.com/DAstringtensionguide.Page?sid=0132cb1d-6b69-4726-b45b-ac76ef91ed3b
Maybe some resizing of the saddle-slots is necessary