If I were to move my Deftechs (aluminum tweeter) to surround duty and the new fronts and center have fabric tweeters, would there be an audible difference in sound between the fabric and aluminum tweeters?
Not unless the crossover voicing is different in the midrange, which is what it boils down to.
The difference between metal and fabric might show up at higher SPLs (which surrounds won't get anyways), higher frequencies (which surrounds won't get anyways and even if they did would likely be too far off axis for them to reach you as 1" tweeters tend to beam > 8khz) but isn't likely a big deal with current state of the art drivers. They're surrounds at the end of the day.
Now the issue with surrounds is something much more problematic than just the brand of speakers you use.
The issue, is that wherever you sit besides the sweet spot, the surround speaker closest to you will dominate response. Sound falls off at around 6db for every doubling of distance, so the difference in level between your two surround speakers is the #1 issue. You will likely be close enough to the surrounds that every shift in seat makes that level difference all the more significant (more so than even your stereo mains).
The problem is that for surround sound, we rely on level similarities to experience spaciousness. If the nearest speaker dominates the surround field, then the results are mediocre - the surround field really just falls apart into "sounds coming from the speaker"
The solution extends well beyond "using the same drivers".
You need the seats to get the same SPL balance of sound from both surround speakers in the upper mids/treble... and not just the sweet spot because when it comes to recordings with surround, IE movies, you want everyone to be enveloped by the surround field.
So to answer your question after my long tangent:
There's a
lot more to it than the difference between metal and silk. You could have identical speakers and even still have issues because surrounds are normally placed above ear level, so the tonal balance is affected by lobing between the drivers. There's also the ceiling reflection which changes the tonal balance again. There's the fact that sounds change timbre when firing directly into our ears compared to coming from in front of us, and again when coming from behind us. If you really want to maximize the surround experience based on , it could be argued that the surround speakers should NOT be the same as the front speakers - but they should have a similar tonal balance at the listening position. Your best bets for surround speakers are large coaxials (IE 12" or larger, aimed diagonally down at the farthest seats) and true floor-to-ceiling line arrays (only a 3db rolloff for every doubling of distance) and I wouldn't be too concerned about <100hz or >8khz performance.