A friend has loaned me a USB microphone, I have the software downloaded, but I don't have a suitable means of getting sound from laptop to system without a bit of messing. That's the main source of delay outside of nappies, etc. I'll get there.
You're just doing some basic room measurements, right? Grab a headphone to RCA adapter and use your laptop's headphone jack.
Edit: Just saw that you don't have a preamp. That does complicate things a bit.