If there is no technical problem such as something with your ac or one of your components, then it could simply mean that you have a poorly matched system that once warmed up just doesn't sound good. A system's sound will change when it goes from being turned off and cold to being on for a while and warm. Some never turn off a system because of the warm up process unless they have a system with a tube amplifier. You do not provide any information on your system. It could be the basic issue of amp-speaker matching (the foundation of any good system, imo). Putting together a good-sounding audio system with a separate amp, preamp, sources and speakers can be a real challenge. It requires research, auditioning, listening, trial and error, as well as simple luck sometimes. You can hit and you can miss.
