Again, not meaning to sound difficult, but everybody has a different perception of "flat". Truly flat usually equals fairly expensive; it also tends to equal "hyped" in the ears of just as many people who hear it as "flat".
I think you should start by asking yourself first just how is it that the translations are going awry? Is there a pattern? Do they all tend to be too bassy? Too midrangey? Too "close" or "busy"-sounding? And so forth.
Then the second question would be if that can be addressed with what you have? IOW, can you adjust how your mixes sound on your mixing speakers to compensate; e.g. if there is too much midrange, can you simply mix with less of the problem freqs on the JBLs to make them sound right elsewhere? If it is a problem with bass (either too much or too little) are you sure it's not your speaker position or room modality that's causing the bass errors? And so forth.
If you've done what you can with the JBLs and the room, or you just want some new monitors no matter what, then you need to go listen to others *with your specific issues in mind*. Take your mixes with you on CD, and when you hear something that sounds like what they sound like not on your JBLs, but in the real world, you're getting close. Then switch to some commercial CDs that you really, really like the sound of and play it on the ones you've already picked as semi-finalists. Then the ones that sound the "flattest" to your ears are probably what you'd want to try.
Finally, make sure you're getting them from a place with a reasonable exchange policy. Keep all the boxes and packing when you get home, and try them in your own room. If they seem to work at least better than the JBLs, keep em. If they suck in your room, exchange them for the next choice.
G.