South,
In my system i found the bugle boys were a poor choice as they sounded a little flat my system is a little dark. The mullard (english not holland) branded RCA provided better dynamics and extension while creating a 3d sound stage and natural tone the sovetechs are not capable of(in my experience.) These tubes however tend to be microphonic and may not work for you however I have substituted many and always return to the mullards.
In my system i found the bugle boys were a poor choice as they sounded a little flat my system is a little dark. The mullard (english not holland) branded RCA provided better dynamics and extension while creating a 3d sound stage and natural tone the sovetechs are not capable of(in my experience.) These tubes however tend to be microphonic and may not work for you however I have substituted many and always return to the mullards.