You're right, if the speakers work one at a time they are OK.
The next thing is to swap the left and right connections between the preamp and power amp. Again, if the problem switches to the other channel, both channels of the power amp are working and the problem is in the preamp, or cables, or something earlier in the "chain".
Assuming you have multiple inputs to the preamp, try switching sources.
And try operating all if the switches/knobs on the preamp because it's not unusual for them to get bad/intermittent contact.
If you need to replace something, the simplest and most economical solution is a receiver. In case you don't know this, a receiver is a tuner, preamp, and power amp in on box. But if you have a turntable (record player) make sure the receiver has a phono input. (Phono inputs aren't as common as they were in the analog days.)
You probably won't get as much wattage as the power amp but you probably don't need it in a home environment unless you really like to crank-it up loud.