Hi, to practice nowadays, i use a little 9V orange practice amp. When i use a 9v adapter with a light on the end, the light gets weaker everytime i play. This dosent bother me, but i wonder if this could affect the sound of other things in my room (like studio monitors etc)? Sorry if this is a ******ed question, but i was a little concerned.
I'm a bit confused but if I read you right it sounds like you are wondering if the 9v AC adapter plug you used with the Orange amp can cause enough fluctuation in your house power to affect the sound of your studio monitors?

I seriously doubt it, but if that were to be a problem you can always get a power conditioner like a Furman.
I wouldn't be concerned. You probably have an unregulated adapter. If you measure the voltage it will be higher than 9VDC. Maybe 13-15VDC. The voltage comes down to 9VDC when you apply a load. When you play the power section of your amp draws more current and lowers the supply voltage.