(If you’ve played Xanadu Next, you may recognize it.) Its 1986 expansion, Xanadu Scenario II: The Resurrection of Dragon, is especially notable for featuring the first work of the legendary Yuzo Koshiro, alongside Takahito Abe. While they were founded in 1981, it was in 1985 with Xanadu (composed by Toshiya Takahashi) on PC-88 that they really got their start musically - while its soundtrack was small, it was the genesis of a main theme that would continue to be used in several later games in the series. I won’t be mentioning every single game or soundtrack, but I’ll go over many of them, and provide samples so you can follow the evolution of their sound.
I did notice a lack of a mention of Falcom in the OP, so I’m here to rectify that with a brief(?) history of some of their work in the pre-Windows era - mostly on Japanese PCs like the PC-88 and 98, but also including some console games. Special album aren’t used in the game itself at all, but they are the originals.
The Legend of Xanadu is a PC-Engine game - it’s just that, as far as we know, these tracks were composed first on PC-88/98 hardware before being arranged for PCE, because that was what Falcom’s sound team was used to at the time.