Since teh Quickie is battery powered, it does not generate hum on its own. The solution thus depends on what the source is. Since it is sensitive to tube replacement, it seems likely to be coming from electric or magnetic fields generated by other equipment nearby; some tubes will be more sensitive to such external fields than others.
You can start the hunt by trying different locations and different orientations of the Quickie. For example, if it is close to a power transformer, move it further away and see if the hum decreased. The second good experiment is to shut off any light dimmers, cell phones, computers, etc. that are nearby to see if they may be a source of interference. I suggest these two experiments because they are easy and lead to answers most often. If they don't lead to an answer then we can try other things.
