Okay, so here's the scoop... Over the summer, I purchased a MIM Fender Fretless Jazz bass. When I got it home and plugged it in, I was surprised to hear a borage of hum coming from my amps. (My rigs: MarkBass 410 with Little Mark II Head, Ampeg PF410HLF with Ampeg PF800 Head, Mesa 212 Powersouse with Orange Tiny Terror, Ampeg 108 Practice Amp...not running any effects.) Now I know that with jazz basses when the pickups are singled out you most always get a low hum, it's just the nature of the beast, but this was over the top. I took it back to the shop I bought it from and had a tech check it out. We tested the bass through multiple set ups and it was quiet as can be, but as a service to me he shielded the cavity and checked the grounds and pickups. When I went back to pick it up, plugged it in at the store and it sounded great. No buzz or hum at all. Got it home...plugged it in...hum and buzz like crazy. So, with no solution at hand after working with the shop for 3 weeks, they gladly refunded my money. Which was very cool of them...and they did all the work for free. Since then, I have had about 5 other basses, 3 jazz style, 1 vintage Fender P, and one Rickenbacker (Brand New From Rick) that have all suffered from the same issues at my apartment. I have even taken my basses to friends houses and local music shops to plug them in and play. Each time I have had no issues at all with any of the basses. For my latest test, I brought my Rick with me to test out some new rigs. I plugged in and played a few Ampeg set ups, but settled on a MarkBass 410 and MarkBass Little Mark II Head. In store the rig sounded amazing. Quiet as a church mouse! Then...I brought it home...BUUUUUUUUUZZZZZZZZ....HUUUUUUMMMMM. A few friends suggested a power conditioner, so I went out an bought a Furman rack mount conditioner...No change at all. At this point, I am stumped...it seems to me at this point to be an environmental issue. Something in or around my apartment is setting off a frequency that I just cant seem to escape. I have narrowed it down to the low mids and high mids however...if I turn both all the way down, the hum goes away. I have also tried different speaker cables, instrument cables, combinations of heads and cabs at my apartment and so on...I have also tried different outlets, unplugging everything in the apartment, etc... The only thing that I can think of is that my apartment is directly above the maintenance shop for our apartment complex and maybe they are running something that is creating interference, but that's just a guess. Just looking for some other thoughts and ideas. Thanks all!