From what I remember the 23.5 go deeper into the bass and if course is the upgrade to the 23.
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I have the Levinson No. 23 and really, really like it. That being said, I have heard that the 23.5 is just a bit "smoother", although I have not heard the 23.5 myself. I seem to recall John Atkinson's Stereophile review of the 23.5 saying that he really liked it a lot, much better than the stock 23. I believe he thought the 23 was not laid back enough for him, especailly the midrange being a bit too forward. (My recollection of the review). I do not find that to be the case, but then again, what do I know? (He's the one with the Stereophile job!)
My opinion is that if you can afford the extra few hundred dollars to upgrade to the 23.5, you should do it.
(If you want, you can email me and I can look up the old issue and confirm my recollection.)
This is some additional information that I had found in an old Stereophile magazine, April 1992 Vol. 15 No. 4 Recommended Components
A significant redesign of the original 23 has resulted in an altogether more musical-sounding amplifier, according to JA (though LL feels it to be too laid-back for his tastes). Compared with the 20.5s the 300Wpc 23.5 offers a more vivid, more dynamic, better-defined view into the image, though it still doesnt approach the standard set by the Audio Research Classic 60 in this respect. Its soundstage is also a little shallower than that thrown by the Krell KSA-250, but it handily outperforms even that beast when it comes to low-frequency extension and weight. At its best with dark-sounding loudspeakers, such as the KEF R107/2 with which it makes a musically synergistic match.
(Vol. 14 No. 9)
I also have the interview of this same amp. In TAS. If you want it I can make copies and mail it to you just e-mail me your address.
Also in the above statement they say that this amp. Is 300 WPC thats an error its 200 WPC.