Firstly, I would encourage you to contact Will Vincent himself.
But my understanding is that his amps are incredibly close and true to the original Dynaco ST70. That being the case, this should not be considered a heavily modified unit.
Most would consider the EL34 more open than the 6L6, as opposed to veiled. Though as you also mentioned thin sound, if you seek more richness and body, you might want to give some consideration to the 6L6.
I would ask what 7199 tubes you are using in your unit? With the right tubes, powering an agreeable speaker, the only cause left for not finding happiness in a Dynaco ST70 would be a difficult room.
But my understanding is that his amps are incredibly close and true to the original Dynaco ST70. That being the case, this should not be considered a heavily modified unit.
Most would consider the EL34 more open than the 6L6, as opposed to veiled. Though as you also mentioned thin sound, if you seek more richness and body, you might want to give some consideration to the 6L6.
I would ask what 7199 tubes you are using in your unit? With the right tubes, powering an agreeable speaker, the only cause left for not finding happiness in a Dynaco ST70 would be a difficult room.