Dolby Digital has 5, 6 (or 7, guys?) different full range channels, with a dedicated low frequency channel. DTS has the same; it just uses a different method of encoding. Both of these utilize digital inputs (see pmbuko's post). THX is a marketing ploy. Dolby Pro Logic has 4 different full range channels (Front left, front center, front right, and surround) with no dedicated low frequency out. It decodes these from the basic Left/Right analog signal. It won't be a waste of money to get the speakers first. Besides, that's what it sounds like you need to replace, first. I had to replace the receiver first, so I went the other route (speakers would not help with a dead left channel). A reciever will not help you with a busted cone.
