Rather than focus on modification nightmares, perhaps it might be helpful to break down your request into specific categories, and rate each service provider on a scale of 1-5, with 5 being the best?
Some categories might include:
Quality of workmanship
Quality of sound
Description of modification to be performed
Service
Communication
Some categories might include:
Quality of workmanship
Quality of sound
Description of modification to be performed
Service
Communication