Well, this is a very frequently asked question, and the challenge headphones, vs. monitors is the eternal one.
Actually all the sound engineers agree (e.g. look to manuals, forums, tutorials etc.) on the fact that a perfect monitors set up, playing back the music "on air" should be the best, because it's a more natural and human experience, compared to the unnatural simulation of the headphone.
On the other side, experts often say that even in professional studios sometime the audio speakers are not perfectly set-up, (very complex to avoid stereo interference, and colour of the room) so imagine in home studio or desktop set-up... then the golden rule should be, try to make it perfect with professional headphone, and it will sound good with monitors as well. Your experience seems to confirm it. I also had the very same experience.
My HD 380 Pro headphone tend to make the high frequencies more evident, and with standard or cheap headphone very often music I considered bad with Sennheiser headset, was sounding pretty good!
Unfortunately sometime I had the reverse experience, and music I considered perfect after mixing with Sennheiser headphone, was not equally good when playing back from my Yamaha studio monitors (very often underlining the bass frequencies instead... I try to follow all the rules for compensation but I suppose that my room can't get rid of the difference, and I simply try to find a good intermediate solution).
So my very personal process is: prepare the mix with my headphone, and later test it with the studio monitors. If it's fine I try it with iPad headset and my car stereo for confirmation.
In regards to Synchron Strings, and high frequencies, I've one more comment: getting old and using headphone too much, usually the user is loosing high frequencies sensitivity. I suppose it is probably happening to me, even if last medical check was still almost perfect 1 year ago. But due to the fact very often are the high frequencies of Synchron Strings disturbing my mixes, I suppose that it's not (yet) age or ears the problem... but I always ask others to cross check to get external feedback :)
