With a dedicated (proper) receiver/amp you can use whatever dedicated sub-woofer you want.
With a HTiB or PC speaker set you are locked into what ever they give you (a matched set). The only exception would be if they specifically allow you to add additional speakers or components as options. To add a stand-alone sub-woofer to a HTiB, it would have to have a "sub-woofer out" port.
With a HTiB, the sub-woofer has to do it's thing as well as compensate for the fact that the small speakers really don't have properly sized woofers. They are minimal systems ... they will never sound as good as a dedicated component system with proper speakers and power.
With a HTiB, I think some of the power circuitry that is normally in the amp part is moved to the sub-woofer box to keep the amp part smaller. Like I said, a matched set. With a PC Speaker set, ALL the power circuitry it typically in the sub-woofer box.
In conclusion, with a little hacking you MIGHT be able to swap out the 5 small speakers (at additional cost) but the HTiB will likely not have enough power to drive them. I highly doubt you can swap out the main sub-woofer box.
Either get a HTiB or PC Speaker set and use it as is, or take that money and put it toward a dedicated component system. Use AVSForum ... the fact that it is for a HTPC doesn't matter because you are using Digital In. I can help you start building it here or over there (just point me to the thread).
For instance, you could get a nice basic amp, one set of bookshelf speakers, and a nice sub-woofer for about $600. You probably have an old set of speakers to use as surrounds temporarily (and a third one to use as a center). After you save some more money, buy a real Center Channel speaker. Next, move the new bookshelf speakers to surrounds, and buy a nice set of main left/right speakers. This is just an example, but hopefully you get the idea. You can build and modify your sound system just like a HTPC. When you buy a new component, buy a good one and you should be able to use it for many years.
Plus, you can use the sound-system for more than just the HTPC.
EDIT: Just re-read your post ... While you might be able to use Digital Out from the PC to the HTiB and leave your exsiting PC Speaker set attached to the PC's 1/8 analog ports ... whether they both will output and work and the same time and whether the 2 sound streams will be in sync with one another is the question. I kind of doubt it. On a properly setup system, the sound is decoded and outputted from one place.
With a HTiB or PC speaker set you are locked into what ever they give you (a matched set). The only exception would be if they specifically allow you to add additional speakers or components as options. To add a stand-alone sub-woofer to a HTiB, it would have to have a "sub-woofer out" port.
With a HTiB, the sub-woofer has to do it's thing as well as compensate for the fact that the small speakers really don't have properly sized woofers. They are minimal systems ... they will never sound as good as a dedicated component system with proper speakers and power.
With a HTiB, I think some of the power circuitry that is normally in the amp part is moved to the sub-woofer box to keep the amp part smaller. Like I said, a matched set. With a PC Speaker set, ALL the power circuitry it typically in the sub-woofer box.
In conclusion, with a little hacking you MIGHT be able to swap out the 5 small speakers (at additional cost) but the HTiB will likely not have enough power to drive them. I highly doubt you can swap out the main sub-woofer box.
Either get a HTiB or PC Speaker set and use it as is, or take that money and put it toward a dedicated component system. Use AVSForum ... the fact that it is for a HTPC doesn't matter because you are using Digital In. I can help you start building it here or over there (just point me to the thread).
For instance, you could get a nice basic amp, one set of bookshelf speakers, and a nice sub-woofer for about $600. You probably have an old set of speakers to use as surrounds temporarily (and a third one to use as a center). After you save some more money, buy a real Center Channel speaker. Next, move the new bookshelf speakers to surrounds, and buy a nice set of main left/right speakers. This is just an example, but hopefully you get the idea. You can build and modify your sound system just like a HTPC. When you buy a new component, buy a good one and you should be able to use it for many years.
Plus, you can use the sound-system for more than just the HTPC.
EDIT: Just re-read your post ... While you might be able to use Digital Out from the PC to the HTiB and leave your exsiting PC Speaker set attached to the PC's 1/8 analog ports ... whether they both will output and work and the same time and whether the 2 sound streams will be in sync with one another is the question. I kind of doubt it. On a properly setup system, the sound is decoded and outputted from one place.