I guess your IEM diversity with that “male and female duet to be equally balanced” part a hint where you may find the ideal curve for.
I’d suggest qdc tuning.
The best you can reach is Anole VX, mid budget options (DUNU SA6 ($560 )& SA6 Ultra (already sold out for $599 as it was 300set limited collaboration item). If a little bit more engaged, See Audio Bravery (OG $279, new shell around the same price)—still one of my endgame for vocal-oriented genre. It’s a bit high In hurdles, but I see a startup in Singapore (Aur Audio) makes Bravery on steroids,Neon Pro for $650. Someone have extensive experience mentioned if properly amped Aur Audio Neon Pro is par or above Anole VX (I’s take it because of slightly spiced up tuning of Neon Pro like Bravery has)
My understanding for qdc tuning is U-shaped with equal mid-uppermid plateau. This will bring you both excitement and equally balanced male /female vocals.
There is a budget option for that tuning, TRI’s Starsea.
If you are seeking more balanced tuning, You may hit Mangird Tea (OG) or AFUL Performer 5, both has very flat expressions across vocal range.
Tea (current $239) is replaced with XENNS Mangird Tea2(299), much smoother feeling of Tea2, in exchange of loss in male vocal range information density. Tea is 6BA+1DD 3 way design. It has TOTL class mid range, as they’ve spent 90% of their budget in midrange. Backed by 4BA of Sonion 2600, totaling 8BA left/right for vocal range, I have to emphasize Tea OG is vocal-beast.
For AFUL P5 going at $220, it has simple 4BA+1DD popular config, 2BA as tweeter, 2BA for mid, 1 DD for subwoofer. The mid-upper range is extremely neutral, but still enjoyable. What makes P5 special is speaker-like thumping subwoofer, which will add realistic touch to baritone region of which male vocal will gain benefit.
Also
@baskingshark mentioned, HEXA is very vocal-neutral as well, with best cost to performance you could reach. A great option to bet. And it’s $79, a fraction of what the other options are offering.