My experience of a Leak ST20 was not warm and mushy by any means - very clear in all departments, almost 'modern' sounding

Quad II's definitely warm and mellow in my experience, a very lovely cuddly sound - but more of an 'old' old sounding tube amp than a 'newer' old sounding tube amp....if you know what I mean (i.e. it's old, not new), so probably not what the quest was for anyhow.