I think your dealer's point is that he feels conditioners limit the power to the amp. Many can (depends). Do you notice any difference, that is what is important. If so, I think you may want to look at something with a parallel line conditioner, where they don't limit the power. You may want to check the Vans Evers line out, I think they utilize a parallel conditioner but also have surge protection, etc.
Regards,
Regards,