My S6 is a couple months old now, about 2 weeks ago I changed the strings using D'Addario EXP coated phosphor bronze 12's (light). All was good.
A couple days ago when I started my daily therapy (practice) I noticed the G chord did not sound harmonic at all, the pleasing musical chord was replaced by a more metallic growl. Very raspy kind of reverb....as you can tell I'm grasping at straws trying to describe it.
I don't think it's me, I was going to try changing the strings to see if the issue continues, but in such a short time I can't see as the strings can be the issue....loose bracing?
As I said, it is most prominent on G major, individual strings sound fine, strumming DGB produces some of this effect, but a full G chord just growls.
Any ideas? Or is it just me? My ears aren't the greatest due to 3" gun batteries but I don't think I'm imagining this change, it's very distinctive.
It's hard to picture the growl but my guess is, it sounds to me like its a saddle adjustment or string and bridge pin. I would loosen all of the strings , make sure the saddle is set in properly then loosen the pins and pull the strings up tight.
For the record I have no qualifications other than fixing my own mistakes. Unfortunately this keeps me busy.
Change the strings. If you play every day they will wear our sooner than you think. If the problem persists with new strings, take it to your favorite luthier/guitar tech.
Hmmm ... On a practically brand new Seagull S-6, I'd think loose bracing would be way down the list of probable causes. The main variable is that you not only changed strings, but you went from uncoated to coated strings. As others have said, it's kind of hard to imagine exactly what you're hearing from a verbal description, but if this is the first time you've changed strings, it could just be the strings themselves, or it could be a problem in how the strings "seated" under the bridge pins. If it were me and the new sound were really bothering me, I'd change back to the original Godin (D'Addario) uncoated phosphor bronze lights and see if the problem corrected itself. If that didn't help, I'd just have a competent guitar tech check it out. It should still be under full warranty if you're the original owner.
Try playing G in some other chord fingerings.
Try putting a capo on the 1st fret, and then try playing some chords. Switch it to the 2nd fret and play an F shape chord, which is then a G - does it have the same growl?
Also try tuning all the strings down a half step, and see what happens.
My guess is that you are getting some resonance from your truss rods.
...When my Maritime first arrived, the truss rod adjustment was VERY loose...(the guitar sounded a little "brassy", and the neck was bowed in a little)...a quick adjustment, and all was well!
All the advice given here so far is good. I'd make sure that the strings are seated well in the bridge and with the bridge pins. There may be one or more that aren't snug to the bridge plate inside the guitar and vibrating obnoxiously. The other thing may be the truss rod being lose. Sometimes the neck may not need adjustment but the truss rod is just sitting there lose and changing the strings may have freed it to vibrate at the frequency of a G chord.