I have found that working with Snarly's EZDome and sIBL HDRIs, you don't need the actual HDR image to provide luminance to the scene. You need to convert the HDR luminance to a 'pure' format. This, combined with the JPG background, illuminates the environment better. Third-party programs are needed to manipulate the HDRs; Picturenaut and HDRShop can modulate the luminance. Unfortunately, HDRShop is no longer available.