Resources on coaching underperforming reps from a QA perspective?

This is something I'm working on too! Perhaps time for me to write a guide on it, ha. 

Anywho, my first recommendation would be to figure out why they are underperforming. Are they undertrained, not invested in the job, having trouble at home? 

The way I look at reviewing their quality, I like using an inverted pyramid model. It allows you to 1) go through reviews a bit more quickly and 2) get very specific in what needs coaching. I've used my magic drawing powers to sketch it out for you: 

You can obviously make this as complex as you want, but the principle of it is this. You start with 3 rounds of reviews, using different scoring templates for each round. You structure them like so:

Review round 1: most top level categories. Think 'tone' or 'resolution'.  
Review round 2: did the agent fail one of the top level categories? Then this round digs into what exactly they didn't do well to fail round 1. For 'tone', for example, you can go with 'addressed customer correctly' or 'adhered to style guide' - anything that is considered important to hit this top level category. 
Review round 3: this is a bonus round for you as the QA specialist. The scorecard here should contain categories that describe where you think the problem is coming from. Is this a matter of additional training? Is it a gap in how you support your agents? Are they missing tools to resolve this problem? 

When you've gone through all these rounds, you'll end up with a very specific list of things your agent is failing on, and what solutions you can offer them to up their game. The beauty of this approach is that you don't have to go through every specific category of all 3 rounds on every review, just the ones that the agent is not scoring well on. 

Hope that helps :)