setup problems on a tension free neck

ive recently acquired a used ibanez usa atk 5 string. the action is just too damn high for slapping and popping. heres the problem... the bass has the bunker tension free neck on it. meaning this thing doesnt have a truss rod. instead it has a 3/8" steel bar going through the neck, and is connected to a brass nut that divides the neck from the headstock. anyway, heres my problem. i put new strings on it, and they are heavier. (DAddario XL's, .050 G string to a .135 B string) and the neck has an insane relief now. theres an adjustment screw at the back of the heel, but i cant seem to get it any flatter than it already is... only more relief. because of this the bass needs a high action in order not to buzz like crazy on the upper frets (19-21). would replacing the strings with a much lighter gauge remedy this problem? like super soft XLs with a.040 G string and a .125 B? would this make the strings really "floppy" though?

 

that is not what i wanted to hear

but thanks.

any reccomendations for the ATK?

 

yeah i know its pretty bad. i just put some lighter strings on it... and its better now... but still not where i want it. id say probably about a 50% improvement. i wrote to bunker guitars (the authority on the tension free neck that they poineered) and i was told to take the steel rod out of the neck and give it a back bend of about 1/8" it seems like its a sound idea, though i dont like the thought of manhandling the neck back into position.

DOES ANYONE HAVE EXPERIENCE WITH A TENSION FREE NECK???

if so PLEASE give me your insight. i am very frustrated.

 

ok so i wrote to dave bunker, and he was very nice and wrote me back. he said that he could make it play like new by giving it a setup... and he wants $125 plus shipping both ways.... doesnt that seem a little...... high??