I perfer my X-230's that I have at home over the Z-4's but most of it I think has to do with the differences in the room. The X-230's had better bass and don't sound as harsh at high levels. Even witht a true 8" sub in the Z-4's, I don't think it matches the quaility of bass the X-230 puts out.
I haven't heard the X-2300 but from what I've read they aren't worth the exrta $60-70. Midrange is almost non-existant in these speakers so paying more for the THX seal and a bit more bass isn't worth it.
Even on Amazon 36% of the people that look at the x-2300 go for the x-230's.
Only 17% actually buy the X2300.