In my experience, it's difficult to get a clean sound and a good saturated lead sound AT THE SAME TIME without a pedal. The cascading gain nature of the amp requires a high Volume 1 level in order to produce a high-gain saturated Lead sound, at least without a pedal. The signal is sent from Channel 1 to 2, so Channel 2's gain is dependent upon the level in Channel 1. I am able to get a good clean sound and a great medium gain, ballsy rhythm sound. I'm not sure what you mean by "heavy rhythm" sound, but it would be hard to get a pristine type clean in Channel 1 and a chugga-chugga in Channel 2 at the same time without a pedal.
The channels, eq and reverb are footswitchable, and Channel 2 has a separate volume level (in addition to gain level), so you have several ways to get boosts when switching either way (from 1 to 2, or 2 to 1). Also, the tone control pots are shared between channels, but the eq can also help here.
I think for what you want, a Mark IV or V might be better. The beauty of the Marks IV and V is that each channel has independent gain, meaning Channels 2 and 3 are not reliant on the level of Channel 1. Also, each channel has separate eq pots, with the exception of Channels 1 and 2 on the Mark IV that share a common mid and bass pot. The Mark V is completely independent eq, and even has many more options than the Mark IV.
Hope this helps.