Today Chris wrote the following:
I checked with Roger and I’ll paraphrase his response. As usual he always gives a very complete and detailed explanation and I am happy to share that with you but this is probably not the place to do so. If you wish to have more information, please contact me thru my website ‘contact’.
In essence Roger does not recommend spending the time/money to change unless you normally play your speakers at very high levels (which is not recommended as you can imagine). The performance difference between metal core and air core is that at high current levels the metal core will saturate and lose inductance, mids frequencies will suffer and have distortion – but only at high levels. Air coil does not saturate. These days many new amplifiers are pushing out much more power and it is easy to push the speaker to limits and even beyond, the air coil prevents any distortion to the mid frequencies, but it is also not a good reason to continuously push the speaker past its intended power rating – the original metal core coil will not saturate if played within the speakers max power rating. You might say this is not so much of an ‘upgrade’ but a move ‘side-ways’. Please let me know if I can be any more help and/or explain further.