What is the height of the room? If about 2.4m (8') it could easily be a height mode. Try just standing up where you are now without changing anything else and see if the problem is helped.
What Steve suggests can certainly help with mode related issues (and SBIR issues to a lesser extent) - but - the 'boom' generally is more of a decay time curve issue where moderate to broadband bass absorption can help. If you turn the room as suggested, just make sure you trap the corner you...
While many of the things above can help in terms of decay time, the first thing is to determine where that 50Hz issue is coming from. If it's a width mode for instance, treating the ceiling and/or corners isn't going to help with frequency response.
Chris Jones - Roadhouses and Automobiles
Greg Brown - Honey in the Lion's Head
Flim and the BB's - Tricycle
Dire Straits - Love over Gold
Tons of others but never forget - whatever you like to listen to. If it won't make you happy doing that it doesn't matter.
Depends on the situation. At shows, many times its new, green gear that needs some times. If it sounds really good at a show, chances are it's broken in a bit. I would agree with the above that I know something I can't live with in about 5 seconds. Something I can, takes a lot longer.