I haven't tried either with IEMs, I only have the Tin T2, I just use those for biking. I can try them with the hel though if you want.
I personally kept the Hel 2, sold the G6. I was able to sell the G6 for the same price I bought it for, not loosing any money.
The volume control on the G6 really bothered me, my headphones were max at 12 volume in windows. Just one wrong button and I could seriously hurt my ears/headphones. Additionally, the Hell just looks 10x better. For something that is probably going to be on my desk for the next 10 years, that mattered too. Where the G6 volume knob was horrible, the Hel volume knob is sublime. The whole unit feels sublime, the switches too. It just feels like a premium product, which is imho, part of the experience.
On sound quality though, I think they are about equal on my headphones (nighthawk, PC38x, KPH30i, Sivga S006), though the G6 sounded a little bit V-shaped, but that could have been placebo. I don't think you can go wrong with either, although I wouldn't pay full price for the G6 personally, the hel is just 40 euro more at that point (I didn't have to pay any custom fees). If you can get a G6 B-stock though, it is a clear winner.
I think someone else should try to see if you get hiss with IEMs, if not, I think the G6 makes a lot of sense if you want VSS