Hello
The difference between the Classical Seat is the rider taking the position on the horse that is advocated by the Classical masters of old. A position that allows the maximum freedom of movement of all its muscles to allow him to express himself to his full potential.
A Deep Seat is achieved by a rider who exhibits a good secure seat with independance of hand and legs. Lots of riders following different disciplines can have a 'Deep Seat'.
I have learnt a lot of the above from my trainer at Bigland Hall Equestrian she teaches the 'Classical Seat' and I recommend others to try a riding holiday there to expand their knowledge.
Bigland Hall Equestrian - Premier Horse Riding in the Lake District