I've tried my 12", 150W Sony sub in my 4,000 cubic foot area. Although it doesn't compete with the 600 for depth, smoothness and SPL, it's perfectly acceptable for filling in the bottom end for music. I haven't heard the 125 but if it's rated 125W average and has headroom for peaks, then I think it should serve you well for music.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated