As Sylvons suggested, mass-killing/cleaving is a good strategy for getting what you want. You CAN still optimize your amber, but the sheer number of chests you will see almost makes capped amber a non-issue.
@OP, I'm not sure why you're RDM/DNC, but that suggests to me that you're meleeing. RDM in abyssea can kill much faster with pure nuking than by meleeing, so if RDM is your only job, I'd strongly suggest /SCH instead. A job that can cleave would be an even better option, if you have one available.
Main thing that seemed to enhance the drop rate wasn't really affecting the drop rate, but chest spawn rate. Check the state of the martello cores.
For that matter, also keep in mind that there is a limited number of chests per zone that can appear at once. If you have decent pearl light and you're not getting chests from nearly every mob, check for other groups cleaving and not promptly opening/destroying chests.