I suppose I've covered this before but it's worth writing about again. Keeping equipment on for best sound and longevity.
When it comes to solid state equipment you'll get longer life out of the product if you keep it powered up. Moreover, you'll get better sound quality if the unit's left on–and it's not just a matter of break in. A constant flow of energy through equipment reduces the shock of turn on surges, keeps capacitors properly formed, and heat levels stabilized.
But are there downsides?
I suppose the answer to that question really depends on the products we're discussing and how they are designed. For most equipment constant power applied isn't a problem. Their heat levels are regulated, parts are chosen for longevity, nothing's stressed to the limits and it's smooth sailing. For others hanging on the marginal edge, like power amplifiers that run too hot, caution should be exercised when it comes to constant use. Tube circuits and mechanical parts, like turntable motors for vinyl and CD transports, are best used only when needed.
For the majority of stereo products such as loudspeakers, amps, headphones, tuners, preamps, power conditioners, and cables, constant use keeps them supple and performing their best, like consistent exercise for humans.
Use common sense, but in the long run, it's best to keep the lights on.