How do you really know if your new speaker, headphone, amplifier, receiver, digital converter or turntable is any good?
One sure sign is that it keeps you coming back for more. If it's great, it may even keep you from checking Facebook or Instagram for hours on end. When you find yourself just listening to your favorite music, without multitasking, that's a really good sign.
Then again, if your new audio purchase bores you after a few tunes, you should probably return it. That advice holds true for cheap ear buds or the most exalted high-end audio toy. If it doesn't breathe life into music or movies, it's not worth keeping around.
Back in the day when I was an audio salesman, that's the advice I gave my customers who were unsure they were making the right decision. In the end, sound quality is impossible to quantify. Sound is ephemeral, so folks get nervous they didn't buy the best one. Some were so overwhelmed by the choices they could never make a decision, and continued to listen to whatever they had.
What other people had to say about a piece of gear was just that, their opinions, the buyer's opinion is the only one that matters. That's true not just for audio, but movies, music, food, wine, you name it. It's your money, you're the final judge of your purchase's value.
Of course when I was a salesman, my customers heard the products before they bought them, and if you're lucky enough to have a brick and mortar store nearby you should do the same.
If you don't have a local dealer and you're going to invest heavily in high-end audio, call a few dealers in nearby cities that stock products you're interested in and get a feel if they're knowledgeable and helpful. If they are, drive or even fly there. It helps if they also use the same gear you have in order to make comparisons easier. In the end It will be money well spent because you'll have some idea of the sound you're buying. In-store comparison listening sessions are invaluable for that.
If you must buy online, read reviews, do your research and then when your purchase arrives follow my advice: listen. I hope you're so happy you stay up late playing tunes or watching movies.