Stefan,
I think you essentially answered your own question. You absolutely do need your survey responses to represent a meaningful percentage of your revenue. To your point, it would be unwise to make strategic decisions based on input from your smaller revenue accounts. Their needs / wants could be very differnt than your top 20 customers.
My advice is to do what you can to convince your top accounts to share feedback. Looking at your website - it appears you serve municipalities. Unfortunately, we don't have much experience with that customer group as our clients generally serve fortune 1000 companies or consumers. That said, some of the strategies we use to achieve extremely high response rates would be applicable. (compelling communication, clear evidence of action taken on feedback, well designed branded surveys, enterprise level software that ensures emails get delivered, support from customer facing personnel, etc. etc.)
Bottom line - you need a statistically valid sample to ensure the feedback data is reliable (and actionable). This not only means a strong sample overall but also when looking at important customer segments.
Side note - have you considered phone interviews in conjunction with email surveys?
Evan Klein
Satrix Solutions
Net Promoter Loyalty Partner
