Nimbostratus
EmployeeA lot of this depends on your exact use case. If you are planning to use your Datapower as a WAF and Load balancer, then it does seem somewhat redundant to place it behind a BigIP. Having said that, I would ensure you are comparing apples to apples. ASM in particular is a remarkable piece of software (yes, I know I am biased) and may well offer features not found in the Datapower.
I would recommend that you look at what you plan to do with your Datapower, look carefully at areas where they overlap, and make a decision from there. You may also want to talk to your account team. They won't be selling you anything, but they can likely pull together some information on how the WAF and load balancer in Datapower compare to ASM and LTM, allowing you to make a more informed decision.