MacBook Pro - 5.1 Speaker system

I have recently received Logitech X-540.

However these only come with analog cables. 3 cables which supply front, rear and centre/sub. I've done alot of searching on this and another mac forum and found the 2 main answers to this problem.

A decoder, like the discontinued Creative DDTS-100.
An AV receiver like this Yamaha receiver.

As I didn't buy the speakers I don't think I'll be easily able to return them for the money so doing that and upgrading to something like Logitech z-5500s isn't really viable.

Are there any other ways to get true surround sound from my MBP with these speakers?
If not are there any cheap AV receivers that people could reccomend?

Thanks in advance
Sven