I have made a few boost boxes over the years for this situation. You can use a power transformer with a 6V-CT secondary (1A rating is fine) and a 3 position rotary switch to have +0, +3, and +6V boost on your powerline. It's not a bad idea to slip a fuse in there too. In practical terms, you can power up the BeePre and just turn up the boost box till the regulator dropout noise goes away, then all is well.