Monster Turbine Pro Gold:
I'll admit i did this more just for the kicks than really seriously sitting down to listen to the two since the 3.Ai is pretty much the obvious choice to go with. That said, i also didnt spend as much time comparing the two as i did for others. Originally i wanted to test the copper but they werent available to me so here's a short comparison of Turbine Pro Gold and 3.Ai.
The Gold...is very colored. Bass is overwhelming, gets quite muddy, and affects the mids at times as well. It is the bassist of the IEMs i tried and the only one that exceeds the bass quantity of the 3.Ai. Comparing the two, it becomes quite obvious that the Gold has what many would refer to the "cavernous" soundstage. Although the 3.Ai has a large bass quantity, I think many people will be pleasantly surprised at spacious Heir Audio has been able to make the 3.Ai soundstage considering the amount of bass it has.
Detail and soundstage goes to the 3.Ai. The Gold has too much focus on bass that, again, it affects the clarity of the IEM. Although the bass of the 3.Ai is big, it remains very clean and does not distort the sound. The 3.Ai does not have the largest soundstage but it does well in a 3D sense and gets the job done.
What surprised me, however, was that of all the IEMs i tried for comparison, the Gold was the closest thing to the 3.Ai in terms of signature, and my guess would be that the Copper would be even closer. The Gold was like a 3.Ai with even more elevated bass and recessed treble.
So in the end, the 3.Ai is similar to the Gold but much more natural and clean sounding. It also gets a significant upgrade in clarity, treble, musicality, amongst man other aspects of the sound. It would be a very nice upgrade for those who like the sound of the MOnster Turbine Pro Gold (and i assume the Turbine Pro Copper as well).
Makes me think that the Miles Davis Tributes are probably a good comparison too. I wouldn't mind a better version of the Tributes myself.