Cube's are generally good for metal, and since the GRX20 has humbuckers you'll be able to get a wide range of metal tones.
Washburn X50 PROFE
Dean Custom 450
Morley PWA Pro Series Wah
Big Muff Rams head
Zoom G2
Ibanez Phase Tone
Randall RG75 G3

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HMMM, well i used to have a cube 60 with esp ec100qm and if you use the R-ifer you can get out a great metal tone for the price, also, try using it at higher volumes for great responsive gain.
The GRX20 was my very first guitar.

Turn up the gain, turn up the tone knob on the guitar