My take: SM3 treble is very good, detailed, and realistic sounding. It does not overpower the rest of the spectrum, and actually does seem like it tapers off. The EM3 Pro has a little more treble volume and the Tesla T1 has much more. Not much more in the way the FX700 had to my ears, but much more. When I compared the FX700 to the SM3, the FX700 sounded like it had an artificial boost and because of that emphasis, did not sound a liquid and natural as the SM3. The CK10 treble sounded metallic to my ears in comparison (as midoo said long ago but I didn't notice until A/Bing with the SM3). The CK90 Pro, which I still have, doesn't sound that way for some reason.
I think many of the "audiophile" or popular IEMs/headphones that are expensive have an either accentuated or very present treble, which is in contrast to the SM3. I think it is something you could get used to if you spend enough time with them and that is the new "normal."
My experience with sources/amps for the SM3 is that the source is the biggest factor. Meaning, the SM3 is ultra resolving and any improvement in the signal path (DAC) will be noticed. Where the greatest differences occur to me is the way the space is presented. For example, I just sold my Rx and picked up a T3. I sold the Rx because, while great with some of my IEMs, I liked the presentation and form factor of the Arrow better and my Rx wasn't getting used.
The T3 isn't as spacious as my Arrow with the SM3. I wouldn't say the difference is large, but for me, noticeable when A/Bing. If I were to grab one one or the other and just listen after not listening to anything I wouldn't know which one I grabbed. On the flip side, if I use my iPod HPO vs. the line out I would notice a difference when critically listening. The differences consist of subtle things like the space between instruments, sharpness of notes, textures, etc. For me, the resolution of the DAC and the amp stage make the biggest difference. Playing around with capacitors, I think that the capacitors (many amps have capacitors in the signal path to remove DC offset voltages of DACs and amps as well as for protection) combined with amp circuit design also make a difference. That is where the improvement lies for me; your experience may vary with different players etc.