I am surprised that you don't find the Metal Zone dense enough. I never find it falling short on gain.
Try adding an overdrive pedal as boost before it if you want more compression in the signal. An overdrive pedal will improve harmonics too. Or try out a DS-1 for crunchy sounds. You could use it as a boost too.
Btw, what amp are you plugging into?
-- My current rig
Guitar: '98 MIJ Ibanez RG520 QS w EMG 81/60
Amp: Laney LC15R Tube, Fernandes 20W SS
Pedals: MI Audio Tube Zone Overdrive, Boss OD3, CS3, MT2, HM2, DS2, CE5, DD7, TU3, Beta Aivin DD2 Delay, Dunlop Crybaby 95Q, Stranger Vol Pedal, BBE SupaCharger
Others: Yamaha RBX 374 Bass, Yamaha MM-6 Synth, M-Audio Fast Track, Shure SM-57, Heet Sound E-Bow Plus
Wanted(pm if selling): EB VPJr/Boss FV500L,Ibanez w Original/Lo-Pro Edge