The 23.5 is an older Levinson amp, but very highly regarded.
I owned a #23 for about 5 years. The 23.5 was a major upgrade to the 23. I have also borrowed a 23.5, and I
currently use a 27.5 which is very similar but lower output.
The 23.5 was a superb amp with great dynamics, solid bass,
good harmonic structure and excellent focus and imaging.
It was rated at 200 W/Chan. The 336 is the latest big
stereo amp from Levinson, rated at 350 W/Chan. I haven't
heard a 336 though the conversation on the Net is generally
positive. However, you can buy two 23.5's and have money
left over for the price of a 336 on the used market. Unless
you needed 350 W/Chan, I think the 23.5 would probably suit
your needs just fine. My 27.5 is terrific.