In my experience (e-mailing concise technical and/or spare parts questions):
EBS - great responses, sevice minded, sometimes a little slow. They really know their products!
Tech 21 - great responses, detailed, all facts, polite AND fast!
Fender - bad. Don't know their own stuff (wrong part references), no replies, idiotic prices on unique parts.
Markbass - friendly, fast, although not managing to answer even a simple question about amount of gain available in LMII vs LMIII. Gave up after two attempts...
Ampeg - quick, professional, helpful and generous.