For some reason, when I turn the gain on the CPB150 all the way up, it feels like playing on a synthesizer. Is it normal (ala the problem of SS), or does it actually have problem?

If it's the SS-related issue, why didn't I notice it before?
Ibanez SA-120 (ed.2006)
BluesJr 1996-B + cathode follower + texas Heat
Crate CPB150
Homemade 4 x 10 cab Bass closeback
Metal Muff