NAS Recommendations? (experienced only, please)

I'm looking for a NAS enclosure for a home Mac network. Mostly Apple devices connected to Airport Extreme. Primary use is direct volume backup -- not primary storage (ie it won't be RAID). As disks fill, I'll swap them out with new ones so need ability to swap volumes in/out when the data is needed (not necessarily hot swap but it would be nice). Any suggestions on solutions you have experience with?

Other criteria:

• expandable enclosure

• 6TB+ capacity

• USB port(s)

• prefer web access

• prefer iTunes streaming ability

...dav