Experts: Why is preamp important?

I know that's a naive question, but the real question is how important a preamp upgrade is relative to the rest of a system. I've heard the statement "a preamp owns the signal", but I don't know what that really means in terms of ultimate sound. For example, is preamp to amp like a transport to a dac, whereby most people would contend that a great dac with a cheap transport can still sound great? I've upgraded my front speakers in a HT system to B&W n803s/HTM1. I currently use a 130wpc Pioneer VSX49tx HT receiver. Obviously I can get better sound from better amplification, and I'm considering a separate integrated amp for the fronts. But the question is can I still make a great improvement using the pioneer's pre-outs to a much better amplifier? Where should I really spend my money? Thanks much. This forum has been tremendously helpful, and I'm sure this thread will do the same.