The Siemens gray shield CCa tubes are considered by many to be one of the best 6922/E88CC ever made. That doesn’t mean that you’ll like the sound they make in your headphone amp or that you’ll hear how good they can sound in a top tier component.
I’d say hang on to them if you plan to own a top tier component one day and experiment with good but not top tier tubes in your headphone amp. This will tell you how good your headphone amp is at conveying the sound of different tubes and how good you are at hearing the differences between tubes without spending big $. If after experimenting a while you think your headphone amp and you are up to the task of appreciating the best, roll them in and enjoy!
I’d say hang on to them if you plan to own a top tier component one day and experiment with good but not top tier tubes in your headphone amp. This will tell you how good your headphone amp is at conveying the sound of different tubes and how good you are at hearing the differences between tubes without spending big $. If after experimenting a while you think your headphone amp and you are up to the task of appreciating the best, roll them in and enjoy!