Yes, it is perfectly fine to leave a solid-state amplifier on. In fact, many amplifiers are designed to be left on all the time. However, there are a few things you should keep in mind if you plan on doing this.
First, make sure that your amplifier is properly ventilated. Solid-state amplifiers generate a lot of heat and need good airflow to keep from overheating. If your amplifier is enclosed in a cabinet or otherwise restricted in its ventilation, it's best to leave it turned off when not in use.
Second, consider using a power conditioner with your amplifier. Power conditioners help protect against power surges and spikes, which can damage sensitive electronic components like those found in an amplifier. A good quality power conditioner will also improve the sound quality of your system by providing cleaner power to your equipment.
Finally, be sure to consult your owner's manual for specific instructions on how to care for your particular model of solid-state amplifier. By following these simple tips, you'll be able to enjoy your amplifier for many years to come!