Interesting replies, thank you for them!
Wonder might that yellowish cover I've got be the orginal one. I see that a little restoration project would certainly be a good idea to this Beomaster - avoiding any problems suddenly appearing caused by the caps or trimmers for example. The "thump" might also get reduced a bit, somehow it feels a bit too strong for me to be fully normal.
Getting inside the device wasn't hard at all indeed, it's just about 4 screws for wooden top cover to get lifted up (in a slight angle and moving a bit towards the back first I think) and 6 very visible screws for the black bottom plate. All those screw locations are printed on the bottom plate - you'll need thin enough screwdriver for the top cover since one of its screws is in a tighter place next to the heatsink (and probably some patience getting it back there if you don't happen to have a magnetic screwdriver nearby like I didn't ). Good news for me that the boards aren't very hard to get out either - oh, I'll need to get my signal meter lamp replaced as well!
All the best, Jouni
Edit: Forgot something - have you had problems with the 75 Ohm connector being way too tight? I can't really get my regular 75 Ohm TV cable fit in (ends reversed, since a radio wall socket has the opposite connector to the TV wall socket). I have to use it the wrong way round (ends not reversed, just like attaching a TV to the TV wall socket). Beomaster 6000 works just fine with the same cable though.