I've been using my VSonic GR99 for at least 1.5 years and I've been very pleased with its sound quality for a sub-$30 IEM. It's been quite sufficient for what I've been using it for, but I've been thinking of upgrading for a number of reasons. The primary reason why I'm thinking of upgrading is because even when using a Fiio E17, the GR99 has a constant, audible static/hiss. If it makes sense, it's that same static/hiss you get when using cheap headphones such as the same static from plugging mid 1990s Gameboy headphones into any form of the Gameboy. I'm fairly certain that it isn't a failure with my setup since the E17 eliminates the hiss I have with Sennheiser HD280s and even when using an Audinst HUD-MX1 the static remains. There's also the issue of comfort.
Comfort is also another one of my priorities. For some reference I found the NuForce NE700X to be the most comfortable IEM by far our of the ones I've tried. While it also lacked the static of the GR99, I also found the general sound quality to be pretty much the same as the GR99. Regardless of how close their sound qualities are, I would have bought a NuForce NE700X if I had to choose between the two now since I found the comfort and elimination of hiss of the NE700X to be that significant. My biggest problem with the GR99's fit feels a little large for my ears and compared to, for example, the NE700X, the GR99's seal isn't nearly as great no matter what size I use. It's also possible that I was putting it in incorrectly and that I didn't find the correct tip size, but the GR99 tip I've been using was essentially the same size as the one in the NE700X I briefly borrowed and I'm about 97% sure putting in a GR99 can't be that complicated anyway.
Currently I have my eyes on the GR06 since I really seem to like the VSonic sound signature. The GR06 ($64 USD) is approximately the lowest I'll go for how much I'd pay for an upgrade, but would the GR06 be enough to beat the GR99 in comfort and lack of static? I'm also thinking of just getting a little more spendy and getting a GR07, but I wonder if I really should go that far or maybe even higher.
Or would something that isn't VSonic suit me better? If it's relevant, my favorite headphone by far is the Denon D2000 so I'm thinking of potentially staying in that sound signature. I originally chose to go VSonic and buy the GR99 for budget and after reading some reviews I thought the sound signature would suit me. As far as I'm concerned, it seems to be just my thing too. If it helps, I also didn't find the RE-ZERO to suit my tastes at all.
Rough budget: Approximately $64 though unless going around $80-$120 is that worth it, I'll consider spending more.