I've been using Xvive U4 for about 7-8 years. If you're having fidelity problems, it's most likely your IEM ears.
I've been running custom Alcairs for about the same time. And I can tell you that the ear phones make a HUGE difference. I started out with decent Etymotics, but the custom Alcairs made a huge difference in keeping the volume of the receiver low. The first thing you need is REALLY good seal to seal out any sound bleed. Definitely customs is the best route, but making sure you have a great (comfortable) seal is a necessity. Next, the higher quality the buds, the more separation you will get, and the lower volume you will need. Remember, you're trying to get out of your IEM, just what you need to execute your part in the show. If you want to experience the show as the audience does, then pull the ear buds and stand in front of the mains.
Here is a quick review I posted on another site about my Alclairs:
I have 3 pairs of Alclair's, play bass and also mix band's FOH.
Dual XB: I started out with the Dual XB, and gigged with them for years. They were way better than any ear bud type IEM's (Etymontic's) I had used, and were comfortable after 4 hours of use. Good solid, middle of the road, but better than any ear bud type I had used.
CMVK: WOW! Huge step up from the Dual XB's. You can practically feel the bass. Everything was ultra clear, while keeping the low end tactile and forward. If I didn't mix for the entire band, I would have happily stopped here, but there was so much bass in the CMVK, it was hard to keep the mix for the entire band. I still use these on gigs where I just play the bass.
Spire Six: Just right... Tons of head room. Keeps the clarity of the low end (CMVK), but brings everything else up. Great clarity and dynamics across all the frequencies. It's crazy how clear and dynamic these are compared to the Dual XB's, especially on stage. I literally find myself turning down the volume on my receiver much lower than I thought, and I can still hear everything. Yes, they are double the money of Dual XB's which were already way better than my Etymotic's.