alt Hacker NewsI've actually messed with Audacity once, extracting the vowels, lengthening them and overlaying, and there is definitely a difference in quality (the length is pretty much the same), it's just that it's very minor to my ear. But native English speakers apparently can pick them up with little trouble.
The difference is much easier to spot in pairs like "bead"/"bid", but that's mainly because before the voiced consonants the long vowel is actually longer than the short one, and the speakers usually add a small glide of "y" at the end of it, so it's more like "beeyd".
I had a native English coworker who used to pronounce it Ü-Bahn and when corrected he insisted that it sounded exactly the same then when I said U-Bahn.
He was perfectly capable of making the same (German) u sound when saying other English words. Apparently it is very subtle, sometimes even for specific things like start of the word and length of the vowel. I just don't know.