Improving User Experience for Enterprise Software

The company I work for creates enterprise software used by everything from multi-national corporations to small non-profits. Most of our interaction with our clients whether it be through sales or customer support is through the managers who purchase the software and not necessarily the people who actually use it on a daily basis. Most of our feature development (reasonably so) focuses on feature those managers want and most of our customer feedback is filtered through the managers as well.

How can we do UX research or other improvements if we don't have access to the individual users?

(NOTE: I understand that the justification we have to make is improved ROI based on improved user efficiency but getting to that efficiency is my question.)