How can I increase the gain of T1-M to better suit ipod or similar source?

Increase R10 and R11 on T1-M will increase the gain. We use 20kohm now, if you increase them to 40k, the gain will be doubled.


How to increase gain on T2?

Increase value R3 and R5. We use 20kohm now, if you increase them to 40k, the gain will be doubled.


Can i leave one channel unused?

Yes


I have 6 ohm speakers, which amp should I use?

Both the 4 ohm amps (T2/T3) and the 8 ohm amps (T1/T4) will work. There are different opinions on which of these amps sounds best, it might be that a speaker matches best with a particular amp. 


Can any of your amps drive 2 ohm loads?

No, that might damaged the amp.