The THR tries (supposedly) to model all-tube amps. In such an amp, distortion is produced at (at least) two different spots: in the preamp and in the power stage.
- Preamp overdrive is mostly used for long-sustain lead sounds. This requires very high gain in the first stage, but not necessarily very high volume.
- Poweramp overdrive is an inevitable side-effect of making an amp loud - basically, of reaching its power limits. This is a more dynamic distortion, good for rhythm parts.
The “crunch” model of the amp probably focuses on the power stage, i.e. the preamp doesn't have so much gain that it really distorts by itself, it merely makes the signal really loud so it can then overdrive the power stage. But that will only actually happen if you also turn the volume way up, since the volume control sits between preamp and power stage.
With real tube amps, in particular full stacks, this is often a bit of a problem: the amps only sound good if played brutally loud, i.e. so loud that nobody else in the band can hear anything. With the THR5, don't know... but apparently that has gain, volume and additionally master controls. For a crunchy sound you'll want to turn gain medium to high, volume pretty high, and only the master down again if necessary.
Note that it's a hallmark of a tube amp to give you a good warm overdrive without sounding very obviously distorted. This is unlike analogue transistor amps, which tend to sound pretty nasty already in crunch setting.