I asked for this to "apple care Italia" this morning, but they said to me that "is not possibile to use one airport express wireless without a second apple base plug in a cable in a modem"
But obviously it isn't true!! i've been made it since i brought my AE... They only want to sell the most expensive router ever!
Good news
: half hour ago i tried to disable "ipv6" from the AE and now the system seems to be more stable; i made also two another change, but i don't think it really matter, anyway: i cancelled psw for airplay on Ae setup and i moved the station Ae nearest to the router (the signal from good became excellent)... So:
1) from the iMac airplay seems to work well;
2) from the i pad air (iOS 8.1) not so well but works, now i'm going to try to switch off... Ok it works also after the restart.
3) BUT the ipad mini (first model updated to iOS 8.1) doesn't show the airplay icon and i don't understand why: i tried to turn off it and to "renew" the lease of the connection wifi, but nothing to do!