Regarding the original question: Pan them the way you want them. This also goes for other bass instruments as well (timpani, bass drum, etc.). Then, on your master bus, merge all bass frequencies to mono. There are several plug-ins that can do this. Search for “multi-band stereo imaging” and you’ll find them, including some free ones. Some plug-ins by Brainworx have a built-in mono maker slider that does this as well.
Merging all frequencies below (say) 120 Hz to mono will not degrade your stereo imaging, and will have several benefits. It will also get rid of the volume balance issue between left and right that you mention.
The upper harmonics of these instruments will still come from whereever you panned them, so they will be perceived in their proper seating positions.
Hope this helps