Mark Levinson 27 or 27.5 The Difference ???

The 27.5 is day and nite better than the 27
ditto the 23.5 v the 23
the 26s v the 26
the 25s v the 25
the 20.5 and 20.6 v the 20

In most cases it's due to teflon PCB's and better board components, however there are some re-design issues too depending on the model. The main effect (with the amps) is more detail, better soundstage, and especially a lot more air or openness to the sound. The prices (original and used ) reflect this also.
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The 27.5 has much more midrange presence, detail, and extension IMO. The 27 is a very fine amp but it depends on the speakers and the sound you want? I liked both of them but in a past system I much preferred the 27.5. In my present system when I owned the 27.5 (again) I didnt like it as much. I was using a Hydra 2 with a Taipan Alpha powercord feeding the Hydra and I had excessive midrange energy that was too much for me. For some reason the 27 sounded better but the 27.5 had better bass. I sold the Hydra and my system had better balance. Part of the reason could have been the Taipan Alpha powercord was too lean and energetic for my taste coupled with the Hydra. My friend thought the same thing.