Sorry to say, but I have the Superlux 381 and 386 as well, and neither will work for monitoring purposes. I wasn't a fan of either one. The Havi however is one of the more accurate iems I have heard. Only a frequency response graph would verify it, as I could be wrong.
The review of the Pistons also shows a V signature, but a good one. We'll see.
I would like to see a monitoring thread. People don't really know how well a flat response with low distortion makes a ton of difference. The Sony V6 is the only truly trusted monitoring headphone out there, and there has never been any mention of a monitoring IEM anywhere to my knowledge.
The S500 is another U signature headphone I ended up not liking. Too much sparkle in the treble.
I'll let you know how the Havi and Piston compare.
I am also not a proponent of burn-in. A driver's signature and characteristic will not change from hour 1 to hour 100. It may possibly "tighten" up over that time, but there is no evidence it will do so. Your perception can vary greatly from every listening session, to every different song though. The Havi has been the same from hour 1 to hour 40 as of now, and I loved it since the first listen. It has not changed since.
I may just keep one or two other iems along with the Havi, as I feel I don't need to spend much more money having the HE-500, the Havi, and an exceptional DIY speaker setup.
