ive found that my 1X15 has plenty of low, but that means i need to back off the low on my EQ to keep it from farting. it also results in a crispy/snappy high. i've played through just a 4X10 and it sounds thicker and fuller and the top is still there, but it doesn't have the same snap to it (which is something i don't like and EQ out on my 15). i found the 4X10 doesn't have the cut that i need in my current band and that the low end gets lost with the guitarists tone so i'm leaning toward the 15. not that i couldn't EQ things to compensate, but the single 1X15 seems to fit in better at this piont. from the discussions i've had, it seems to be the low-mids that give the 4X10 the muscle and in some cases
i've also noticed with the 1X15, it works better for cleaner/low-gain tones, and i'm not a fan of the way it distorts, with either preamp distortion or volume induced-power amp & speaker distortion. the 10's seem to have a smoother distortion and can take more volume before they fuzz out.