"Still, I find it weird that the back jacks don't allow this. I guess they really only want a line level which my stereo probably does not put out."

not weird at all, as you said they're just unbalanced line inputs, therefore you need to control the level at the source. They expect to see a line level, so if you're running anything else, like a powered pre-amp or power amp, you will overload them. Sounds like you found a workable solution though.

For better fidelity though you may consider getting a phono preamp that converts to spdif digital, the converters and preamps on the firewire 410 are less than ideal (I had the 1814 for a while, same box basically)