$1500 is an exvellent price for a 23 (if it's in 8/10 condition) but you'd have to pay at least $2500 for a 23.5.
I owned both and can tell you the 23 is an excellent amp but no better than many dual-monos in that power class. The 23.5 is in a class by itself -- the smoothness of a tube amp with the bass control and high end extension particular to solid state. Here's an excerpt from Jon Atkinson's review in Stereophile:
While I was not a big fan of the original '23, finding its sound to be too vivid, I noted in my review (Vol.14 No.9) that the No.23.5 offered a more dynamic, better-defined view into the soundstage than the '20.5, which by comparison was too soft and rather veiled in the treble. The older amplifier's upper midrange/low treble also tended to sound rather grainy, particularly before being fully warmed up. With the No.23.5 outperforming it, it was obviously time for Madrigal to rethink the No.20.5, the result being the Mark Levinson No.20.6.
.
