This is one area where Macs do seem to be a bit odd. The digital SPDIF output is designed to be used with amplifiers which can take a digital signal, such as home theatre receivers. The speakers you’ve bought are budget low-end things, which have the power and amplification stuff built in, thereby negating the need for a separate amp.
Dedicated 5:1 systems that will work with the SPDIF output are pretty spensive. Bose do some, but they are pricy. The Logitech system Count mentions is spensive too.
Personaly, I run standard analogue stereo into an analogue amp, then out through normal speakers. The sound quality is betterer than all but yer spensive home theatre set ups, anyway.
But anyway, seems like you’ve found the solution. Mind telling us what gizmo you’re buying, for future reference? Ta.