Jaytea,
Since I have not heard your ATC-3 pre in any system, I don't know what difference you may hear. If you can manage to get a 300b to put into your system and compare, this would be the best way to find out. I took a look at your system, and running a manley steelhead into my 300b is something I wanted to do for some time. If you do manage to find a way to get a 300b to try out, you won't hear all it has to offer with the stock tubes. This is where Manley and I differ in opinion, as they feel 'tube rolling' is not going to upgrade their equipment. Not sure what 300b tube is stock these days in this pre-amp.
Since I have not heard your ATC-3 pre in any system, I don't know what difference you may hear. If you can manage to get a 300b to put into your system and compare, this would be the best way to find out. I took a look at your system, and running a manley steelhead into my 300b is something I wanted to do for some time. If you do manage to find a way to get a 300b to try out, you won't hear all it has to offer with the stock tubes. This is where Manley and I differ in opinion, as they feel 'tube rolling' is not going to upgrade their equipment. Not sure what 300b tube is stock these days in this pre-amp.