As long as you understand that the Gradients are WOOFERS not subwoofers you'll be satisfied by their performance.
They were designed to be used as bases and work very nicely that way for a single pair of Quads.
Stacked Quads? You may get better performance from a single set and the Gradient. Stacked Quads only buy you more dynamic headroom at the expense of imaging and room placement flexibility. With the right amps I never felt that the Quad 57's needed more radiating area so the need to stack them was moot. YMMV...
They were designed to be used as bases and work very nicely that way for a single pair of Quads.
Stacked Quads? You may get better performance from a single set and the Gradient. Stacked Quads only buy you more dynamic headroom at the expense of imaging and room placement flexibility. With the right amps I never felt that the Quad 57's needed more radiating area so the need to stack them was moot. YMMV...