help with high quality balun/m

I' off the newest best thingee to the oldest best thingee. I have a reconditioned Marantz 10b that only accepts a 300ohm antenna. I have an outside antenna and cable but both are terrible with the old balun/m that I had on hand from G_d knows when. Some basic research tells me that balun/ms are basically inefficient, especially the cheap ones. So, if any of you golden agers can lead me to the holy grail of balun/ms, I would be a much happier listener.

Denny Daniels