Why can I only access 3 of the 4 usb busses on a mac pro?

I'm attempting to aggregate four cheap 7.1 usb soundcards to give myself 32 channels of audio output (I have good reasons for doing this). It seems that the devices cannot be differentiated when they're connected to the same usb bus (I'm not sufficiently well informed about the devices to know why this is) and thus the aggregate device fails.
I've been able to connect three of the devices to a Mac Pro (specs below) without doubling up on USB busses (giving me 24 working channels). Theoretically the Mac Pro has four busses (confirmed by this thread: https://discussions.apple.com/message/9053084#9053084) After attempting various combinations of pluggings-in, however, I find I can only access three of these (therefore can only run three of the usb devices without them doubling up on a bus). One of the four busses remains perennially empty. Is there any particular reason for this? Is the fourth bus only accessible internally?  (Again according to the thread above it would seem not, but I still can't access it). Does it have something to do with high vs low-speed busses? The soundcards are USB2.0, so maybe that MacPro can only handle three high-speed busses...
I suspect that I shall have to resort to buying a PCI USB card. Which prompts another question: do such cards offer independent busses for each port or would all ports be routed to the same bus? My guesses err towards the latter or a combination, and that it differs from card to card. But I can find no information in the specs of any of the cards I've looked at which tell me.
Rather obscure questions, but I'd be grateful for any help.
With thanks,
pete
Running:
2x2.8GHz Quad-Core Intel Xeon
4Gb memory
Mac OS X 10.7.1

Maybe it's the total power requirements of the attached USB devices and you need a powered usb hub.

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