Line Stage Tube Replacement for CJ ET3

This is more of a comment than question but I thought it bears mentioning to the community for comment.

I have owned an ET3 preamp for about 7 months now and about 5 months in, I noticed a high frequency hiss coming from the right channel. I emailed CJ who promptly responded with a replacement line stage tube which they sent me for about $60. Great customer service by the way.

Before I go on any further, I have to confess that I was never that impressed with the ET3 or the sound of my system in general. It always sounded a little "off" although it was difficult to pinpoint what the problem was. I played with speaker placement, cables, audio sources, etc. Nothing made a huge difference. I was left with a sinking feeling that I spent a lot of dough for ho hum sound. I was also regretting going with an all-tube setup (amp and preamp).

With a newly installed line stage tube I turned on the system for a test and was immediately floored. My system went from mediocre to stunningly incredible. It's been several days with the new tube and I'm still in shock. Every track I've played sounds amazing!

How many people have had this same experience? It slays me to think that I spent thousands on a new audio system only to find that the entire experience was at the mercy of a $60 part. Furthermore, without the hiss, I would have never thought to email CJ, prompting a replacement tube.

So my overall question is this: How does one know if the tubes in an amp or preamp are in peak condition other than a hunch that things sound "off"? How many of us are settling for less than great sound because the new gear we purchased came with defective or rapidly degrading vacuum tubes?