From the standpount of cost effectiveness, there would be nothing wrong with using the 150w amp for the speakers, I totally agree. But I disagree on the idea of a bigger amp being overkill!
plus, Deaner's own words were it doesn't "provide the power that I was hoping for".
I severely doubt the amp puts out 140watts RMS anyhow. In fact, on the assumption that it doesn't, it may even perform weaker than 50w per channel head unit. To me, 400 watts sounds ideal for a starting point for basic speakers, especially if you remove the lower frequencies from the audio signal.