You've already got the Bassman 10 and the the 59 reissue; if you're not emotionally attached to the Super Twin, I'd sell that and get an amp dedicated to metal. While some amps can try to cover both, I think you've got Americana covered with those 2 Fenders; why not get something else with a 4x12 that is purpose built for metal.
Just my $0.02.
nedorama Monkey Boy Studios
Summit 2BA-221, TLA-50
mBox Pro 3, Pro Tools 10.3.4, OSX 10.8.3
Radial JDI x 2, ProD2, ProRMP
'65 Bandmaster, '65 Tremolux, '65 Showman; '74 Princeton, '77 Princeton Reverb, Dr. Z. Mini Z Head & 1x12 Cab, pedals, George L's cabling
'65 Farfisa Compact Duo www.facebook.com/thetoysband