I'm glad you found a better way. Hopefully you can adjust the amount to your liking. Just cutting off above 10k does nothing for the remaining hiss. Background tape noise actually exists in full-spectrum so you would still have all the noise below 10k to deal with.
There are harmonics that can reach into the 20KHz range, but whether the tape used can go that high or if your hearing can detect that or not is another issue
