I was chatting with a friend about this...he brought up a couple of things - perhaps either your preamp (or the tubes in it) is lame-sounding and/or your amplifier has no 'guts' into these 4-to-5 Ohm speakers. While I personally think your McCormack would be fine, I definately agree with evaluating your pre-amp.
Also, do you have the Callistos pulled out from the wall behind? If not, the loud voice-range reflections from that wall makes the speakers sound mid- and upper-bass deficient, something which the subwoofer cannot help. A TV screen or a rack of stereo gear placed right between the speakers has the same effect.
Also, did you purchase your Callisto's new (and register your speakers)? If so, make sure you receive your Happy Ears System Test CD from Roy. It's nearly 80 minutes of music to help you maximize the setup / configuration of your system.
Again, I do think your McCormack amp is more than adequate, but my friend's experiences with your preamp were never overwhelmingly positive. Hard rock / metal and dynamic usually do not go hand-in-hand (sounds strange, I know). Look at (listen to) the latest Metallica release as a *great* example. It's loud, but there are no dynamics. Throw in some Dead Can Dance or Sisters of Mercy "Floodland" (or one of my personal favorites for evaluation - Tool's "Lateralus") and evaluate.