iMac + remote Bluetooth speakers: How to aggregate audio devices?

I have one of those iPod slot on top speakers in a box systems.

I bought a “wireless Bluetooth music receiver” (looks like a chunk of licorace) that plugs into the iPod slot so I could transmit A2DP V1.2 audio to that speaker box from the iMac.

It works and I can transmit audio to that reciever and hear music from my iMac via iTunes and Pandora on the web.

But I want sound to come from both the built in iMac speakers and the external speaker box at the same time (kind of like having close by remote wireless speakers).

Using Audio MIDI Setup in Utilities, I created an Aggregate device so that I might play music through the iMac speakers and the Bluetooth speaker box at the same time.

(For those unfamiliar with Aggregate devices, please read here for quick intro. Note that when you search aggregate devices, many of the answers/links show a previous version. I think pre Leopard or Snow Leopard/Lion version. In current Snow Leopard/Lion version, you add by clicking the plus sign.)

No matter how I have the Aggregate device settings configured, it will only play to one or the other devices.

Is this a hopeless cause or, is there a trick to get sound out of my iMac speakers and the remote speakers with the Bluetooth reciever in the iPod slot at the same time?

I sound like an audio pro but I am not. Indeed, I still can't find an explanation anywhere on how to know your best Clock Source, what a clock source is, what a resample is, why it is needed, and while I have matched device formats at 48000 Hz and 44000 Hz, the 2ch-32 and other bit numbers never match and, does that matter?