How much weight can an upstair room can hold?

You should have no problem. The heaviest item per square inch in your home is your refrigerator when full. It's even heavier than a waterbed per sq inch. There are lots of homes with frig's on the second floor. I have my record collection and stereo on the second floor of my house. My house is over a hundred years old. I did add support under the records but not the rest of the room. 400 lbs is about 2-3 people at most BTW.

you could easily get a ton of people (just 10 to 15 persons) in that room and not think twice about it.

I would not worry...

My entire system is on a 3rd floor and it weights way more than 400 lbs.The rack alone is probably 300lbs(sand fillable coloums)then I got the Rogue Zeus at 200lbs plus 2-3 other heavy amps,cd players,records,cd's,other gear,furniture,name it and it's up-there.I wouldn't worry about it.When they frame floors ,they take into consideration the live load and dead load.Beams and stringers deflect ,they don't just suddenly break.If you see hairline cracks on the ceiling below after you move your gear upstairs then you know you got too much load,but I believe you should be just fine.
Enjoy your system and the tunes!
George

Older house has an edge because old trees are denser and can support more weight.

You can always spread out the pressure point by placing a platform under your rack, can act as vibration damping device as well.

Semi,
I would check the distance between joists if I were you.
They used 24" centers alot and the joist is usually a true 2X6 in older houses. Today it's 16" centers and the joists are 1.5"X7.5" at best. The issue is, will 24" centers with 2’X6”s hold enough weight? Maybe, but when I remodeled my 3rd floor they made me add joists between the existing joists and onto the existing bowing 2"X6" joists to bring it up to code.