I use 1964 Audio V6s (aka, V6 Stage) 6-way custom molds.
They were about $650 back when I got them. These days, all their custom stuff is much more expensive (and fancier).
Not flat by design... And to an extent I regret that choice. I only found 1 brand that offered a "flat" IEM (Alclair) at that time, and very little info on them so I decided on the 64 Audio instead.
I like IEMs for things like individual mix, consistent sound no matter where we play, etc... But I don't really "enjoy" playing with them as I prefer to hear guitar "in the air".
My band has been 100% direct for about 4 years... There is no stage sound (except vocals).