Ah. So just noticed this is also the case on the 10w practice amp. A nice electric shock once again, this time enough for me to reel my hand away quickly and uncontrollably and into the shelf, and I now have a nice cut on my wrist. Fantastic.
So it's both amps and all basses - so likely a problem with the plugs. I will dig deeper when I get our mains socket tester back (brothers had it since he moved out). And since I inherited his room I want to know whether it is a problem located to this room, part of the extension that the previous homeowners had done, or whether it's housewide. I do however find it odd that I've only noticed this since a few weeks ago, despite having the amp for nearly half a year, and not having any troubles with the old one ever until now (had it for 8 years)...