MCD550 with mc275

I have an MCD550 and looking to get mc275. 
MCD 550 has a very nice DAC. 
Would I need a preamp or is it possible to connect mcd to mc directly? 
I don't plan on using any other sources besides CD player, perhaps digital source at some point in the future. 
What is a function of a preamplifier beside providing a DAC and volume control? (which mcd has). 
Currently I have my MCD connected to MA6600 integrated, and the sound is is very pleasing to me. However, I would like to try tubes. I listen to mostly classical music at moderate to high levels,  very dynamic,  is this a good idea? 

My questions: 

1. Do I need a Preamp
2. Can I use mcd directly to mc275
3. What is the function of a preamp in the above scenario? 
4. If adding a preamp is helpful, please explain how?
5. Which one would be better fit for mc275 c22 or c2500
6.. If I choose a preamp with a build it DAC, how would that improve/worsen sound from MCD dac? 
7. If I get C22, would I need an additional DAC if I add a digital source to my system? or can I somehow run it through the MCD without sacrificing sound quality? 
8. Is it a good idea for classical music