1. The first thing I do is turn off amp & preamp and swap the amp phono cables at either end. Now listen again. If the problem stays in the right speaker its not the preamp
2. grit your teeth and swap the speaker leads at the amp. Now the amp L channel is driving the R speaker and vice versa. If the R speaker is still out its the speaker or the R speaker cable. If the L speaker is out its the amp R channel.