HI,
I feel your frustration. When I first started out with the MacMini I was not sure how to get the best sound out of it either. There are several ways to go and each step gets more complicated and expensive.
So here is my take on the subject.
1.) THe Mac Mini has AU audio which is quite suitable for two channel Hauptwerk free, basic or advanced. The Headphone jack is used to access the audio and an adapter cable is used to go from the 1/8" headphone jack to the 1/4" TipRingSleeve (TRS) jacks for the powered studio monitor speakers. Hauptwerk will automatically use this output when you install it so there will be few if any adjustments to Hauptwerk. To add a subwoofer you simply route your cables to the subwoofer and then use other cable to connect from the subwoofer to each of the speakers. The sub will send the audio on to the speakers as a normal condition. You can also use a 1/8" to RCA pin plug adapter and plug into your home theater system but it will be stereo not dolby.
2.) After you have the Advanced version of Hauptwerk you will have access to more than just two channels of audio in Hauptwerk but it gets more complicated with both the hardware setup, (which will require a multi channel sound module and multiple identical speakers) and the Hauptwerk set up. So I believe that to start off you should do paragraph 1 first and play a while before you go to try to go to 6 speakers and a subwoofer.
Here is the process step by step.
Step one is locate the headphone output. Get Hauptwerk working with headphones theough this jack.
Step two is purchase an audio cable(s) to allow you to put your speakers in an appropriate location along with two powered speakers like the Behringer, M-audio or JBL LSR305 to put there.
Step three is get this working and the add a suitable subwoofer to the setup. Set the subwoofer to 50 Hz crossover and send your signal to the subwoofer first and then to the speakers.
Step four is play, play, play
Best regards,
Thomas