By '77 he was using a 3X15, a 4X12 and a 2X18.
Blair Jackson, Grateful Dead Gear, 170.
During this period [Spring '77 tour], Phil was playing his Alembic custom through an Alembic F2B preamp hooked up to a McIntosh 2300 stereo power amp driving a pair of Hard Truckers cabinets filled with Gauss speakers - one with four 15s, the other with three 12s.
ibid., 177.
Peter