You mean the Marantz MA-9S2?
IIRC the Luxman M-120a was manufactured in the early 80's. It was the forerunner to the more famous M-02. A lot of Luxman amps of that time were really nice (M-117, M-03, M-05, MX-100). They had a very tubey sound, quiet, silky smooth, lovely highs, slightly rounded and loose bass. Difficult speakers that dipped below 4 ohms could cause a little harshness, but this was rare. You could set two of them up as monoblocks putting out 300 watts per channel. The last M-120a that I saw sold went for $400 in very good condition.
That being said, you're comparing apples and oranges. The Luxman is a vintage amp and uses vintage parts. Even though the Luxman is well-built, the Marantz build quality is exceptional and all the parts are of the highest quality and latest technology. Even taking inflation into account, the Luxman would sell new today for less than half of what the Marantz goes for. I have to admit that I haven't heard them, but I would have to believe that they would be more detailed and dynamic than the Luxman with tighter, better bass.
I have heard the M-120a, M-02, and spent a fair amount of time with the M-117. I have also spent time listening to the Marantz MA-700 monoblocks. Even though I prefer the Luxman M-117, it was close. The Marantz MA-9S2 is in a different class altogether.