Okay, but don't you find 5.1 is better for movies with a 5.1 signal on the DVD to split the signal evenly?
If you try and split a 2.1 signal to a 5.1 signal, of course the bass would go, like if you did it with music, it would be even more noticable since its also splitting the sub against all 5 too.
If your soundcard has a Pro Logic 2 decoder though built in, the problem would avoided since this can decode sound in 5.1 from a 2.1 signal properly and keep the bass too
Power DVD for example, you can buy an audio pack that has a software Dolby Pro logic 2 decoder which works the same way, except its not hardware though can be used for music and movies, and plus, no slowdown
This would only work within Power DVD though, unless you had a hardware decoder that is