Re: Tuning a 12 string?
A well designed acoustic guitar takes string gauge into account. Martin Dreadnaughts are designed for 13's at standard pitch. It takes that much tension to allow a plucked string to actually move the soundboard and resonate. 000 sized Martins are designed for 12's. Lighter strings don't move the top, and start sounding like an unplugged electric. Tuning down does the same thing, reducing tension, and creating a more tinny, small sound. I'd check the manufacturer's website to see what the stock string recommendations are. I agree with BoogieBill, a set of light 12-Strings should be OK tuned to pitch. If you DO want to down tune over periods of non-use I guess that's OK, but when you tune back up there will probably be a short period for it to settle in. (Also, your comment about low action reflects the reduced tension on the neck from down tuning.)
With any quality steel string acoustic a fine balance between tone & durabilty is strived for. It's easy to make a guitar that lasts forever but it will sound very poor conversely it's easy to make one that sounds great, a wafer thin top & no bracing but it probably wouldn't last more than a couple of hours as I said previously:
However a more expensive model will have a lighter (thinner top) & less but more delicate bracing so that the top will vibrate & resonate more for a better tone. how the top moves is responsible for the vast majority of how an acoustic sounds & plays, it's not called a soundboard for anything it's essentially your speaker cone.
ANY string gauge will cause the guitar top (& back & sides to resonate) I personally guarantee a set of 8's will cause the top of a martin to resonate I have not personally owned a martin but owned a guild I had permanently set up in nashville tuning (ridiculously light strings) & it resonated like a bell.
Tuning down does NOT create a small tinny sound in the slightest if anything the opposite is true try tuning to open C or D, hell even try drop D play the open D shape with the new low D string & listen the bass & just the meat of the sound it increases exponentially.
The truth of it is any guitar is a HUGE compromise in engineering design to get it stable & sounding great and the truth is it fails at both there are countless old acoustics out there both 6 & 12 from all the big boy manufacturers with bellied tops, lifted bridges, necks that need reseating. cracked heels, cracking soundboards etc that will attest this.
As I said before a cheaper acoustic is probably far more robust, a quality one should also last a lifetime or two also but it is far less sturdy than an entry level yamaha for example & needs to be cared for as such.
Use a humidifier, keep it in a case, clean the fingerboard frets etc when restringing & detune it whilst storing it
My '76 Guild just now had the bridge pulled up and is in the shop for a neck reset and will fix the bridge as well–if its built well then it will last 
