What other format would they be in, apart from WAVs
My understanding is that music purchased through iTunes is normally in AAC format. (Checks...yup, the three tracks I have purchased through iTunes are all in AAC format.) I can't remember whether the Humax can play AAC. As you say, though, it's easy to convert from another iTunes-recognised format to MP3 within iTunes.
I have no problem identifying where my iTunes mp3s reside. They're at username>music>iTunes>iTunes Music>Music on my MacBook Pro.
Just as well you knew that! I've remembered now that my iTunes MP3s are on an external drive and not in the usual default place that iTunes puts them.
What I wish to achieve is to have anything I play in iTunes on the Macbook to appear on the output of the Hummy, the audio outputs of which are connected to my HiFi amplifier.
As I said, I have XBMC, on the Mac, which sees the Hummy and can playback Hummy-stored files remotely i.e. on the Mac as a stream. I'm happy to download any other application.
I'm unsure how to reverse this and play my MacBook iTunes through the Hummy. If you could jot down an idiot's guide to how to this, I'd be eternally grateful, as I'm sure would others.
As Black Hole said, unless you want to use the custom firmware you have to put a DLNA server on the Mac. Your Humax will see that on your network and you will be able to browse the shared music via the on-screen interface on the Humax. As Black Hole confirmed and I've just found myself, you can't control the streaming from the server end, or from an intermediate device. That means that you will need to have the TV on, at least while you select the track you want to listen to.
Black Hole says that XBMC can be used as a DLNA server as well as a client. I've never managed to get that to work so I can't offer a guide as to how to do it. The software I use is Twonky server
(N.B not free, but not expensive) which I found easy to set up and manage. I just tell it to use the top-level iTunes music folder as the root of its audio streaming service and I can browse my complete iTunes library from the Humax.
N.B. Since I don't have anything plugged in to the Humax except the TV, I can't confirm whether streamed audio is output on anything other than the HDMI port. I'm sure someone else can advise.
UPDATE: The latest version of Twonky Server, which seems to be the only one now available through their web site, seems to crash the Humax. I was previously running an older version which was fine. I updated this evening and it was causing problems, so I've reverted to the older version.