One suggestion I have reluctantly because I have not tried it myself, is shearing pool of your DNA with a Covaris set to 150-200 bp if you have access to one. In this case smaller fragment would not shear any further but larger fragments should shear. You can try shearing a cheap source of DNA to your current peaks. Pooling them to emulate your current ChIP DNA pool and trying shearing on them first to ensure that it works and that also will allow you to optimize if things does not work out at first go.
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