I had the same question over a year ago and received the following replies: http://www.ae5x.com/blog/2010/09/05/the-most-beneficial-k3-upgrade-panadapter-or-sub-reciever/?trashed=1&ids=1987 I've still not added either but after operating the K3 much as you do, there is no longer any question in my mind - the next upgrade will be the sub-rx. John AE5X