Fisher 500B

Quick google search, from Head Hifi

As far as headphone listening, as noted, the Fisher 500B lacks a headphone jack. You should have no problem driving AKG K1000 phones from the speaker taps without a coupler box. If you want to use standard 1/4" headphone plug wired headphones, you can wire up a simple adapter with a 1/4" headphone jack, and a pair of 1/2 watt 100~150 ohm resistors in series with the left and right channel audio signal.

That unit has two Line Out/Line In loops, (tape loop, reverb loop)

Is there a gizmo that can take line level out, and have a headphone jack, perhaps with volume control?

AI quickie

"To use headphones with a device that only has a line out, you must use an intermediate device to amplify the signal:

• Headphone Amplifier: Connect the line out to a dedicated headphone amp, then plug your headphones into the amp. "