The PL400 is a nice amp and you are right, a little refurbing and a protection circuit and it will do a fine job. Definitely better than the Yamaha.

As a preamp stage, the Yamaha will be fine for now but you will be shocked at how much of a difference a good preamp will make. When budget permits, that and a good external DAC will make a big difference.

Congratulations on the PL-400. These are the ones with the mechanical meters and no on-off switch. The Series II has LED meters.

Adjusting the bias can be done by a professional or yourself. If you do anything yourself, there are lethal voltages in the amp - so you better know what you're doing.

If you would like professional service, try contacting Ed Blackwood at callab@frontier.com. He serviced these amps at Phase Linear many years ago. I had him service my amp about five years ago. If the amp is working fine (no blown transistors), he can update the power supply caps, and optionally to larger-valued ones for more energy storage, as mine were done.

The amp is direct coupled throughout the signal path. Putting a speaker protection circuit in may be safer, but will introduce additional stuff in the path. I do not have any speaker protection circuits in mine. Good luck.