The problem for me is I can't check that 4.1 Youtube because I don't have the hardware anymore. I suffered the same fate as you
Up until Christmas I had the Essence ST, but no PCI on the new board so it's just sitting there (along with a USB TV tuner that still accepts all UK channels but no driver). I ended up ditching surround, got a USB dac/amp with headphone and line out and will eventually replace my ageing 5.1 reciever with a stereo amp.
I'm not sure ASUS is in quite the same league as Auzentech though, at least in the driver department, I doubt a new STX will support any more configurations than your onboard - Probably a better chance with the OMEGA eCLARO but that's only a wild guess though. I don't think flexibility is catered to the way it used to be. I'd be 100% certain before getting anything.
Do you get sound from the rear when you use the test function in the Windows speaker settings?
If you're thinking about a soundcard it might be better to stick with the onboard you have and update your powered speakers instead, I haven't had to buy PC speakers for as long as you though, doubt I can be much help.