Well, I'm gonna let you know.
From the 40's, it's the obvious and best upgrade from my point of view. It keeps the same sort of bass but better controlled and minus the boom. It is bassy so you won't lose out if you're into bass. It does have treble but it's so much more refined and not harsh after some hours of burn. On open box, the trebles weren't even close to the 40's for me and I was able to tolerate it. After 30 + hours, it didn't bother me at all. From that point on, its been nothing but pure enjoyment. I use only the silicon tips provided also. As for the 40's, I really did like them, but the treble was just too much (didn't give them 200 + hours and never tried foam tips). I got fatigued after about an hour of use. Since I tend to purchase many iem's/headphones, I didn't see the need for the 40's so I got rid of them. But, no matter what I have now or purchase later, I will be keeping the 70's and 80's because they are THAT good to my ears. I've sold earphones/headphones that cost 2-3x's more than the FXD's because music just sounds more enjoyable on them. You will get a good idea what these will sound like from the 40's, so add refinement in all areas plus soundstage and you have the ultimate upgrade from the 40's.