If I get a tube preamp I could just connect it to my recording interface and microphone with 2 XLR cables?
I guess so, but you don't HAVE to do it that way, depending on the inputs it has, you can use the line in/line out sockets or DI...then again, this totally depends on what preamp you buy and what your 'recording interface' consists of...
Not that i know of...but i imagine if you use XLRs the quality will be very high anyway, as XLR sends signals well. The biggest factor for sound would be your microphone and the way you eq it. I'm sure it will sound great though. I intend to get a tube preamp soon, but i record onto my laptop so i have to connect it all differently anyway. I think i will have to use the audio output to send the signal to the microphone socket on laptop, or something lol