I've got a bunch of amps since I've been on a buying spree lately......... Old Marshalls, Fenders, Vox, Bogner, Two Rock, Mojave, Aiken, Divided by 13. Germino and Connie on the way, and on and on............ I notice that even at equal volumes, some seem to bloom into that great harmonic wailing feedback much more than others. Others will squeel long before they will "wail". What are the critical elements of circuit design and/or voicing, tubes, etc that make an amp sing?