Adjustable gain on the RCA inputs may or may not be a big deal for you. They're primarily intended to permit control of a system with fixed line output (e.g. public address), adjust balance or compensate for a higher-than-normal output voltage from the preamp. The XLR inputs are a nice feature; if your preamp offers them, that configuration can better reject noise and can be useful in certain environments (high EMF or RFI).
Series IV cannot be upgraded to the V/IV style speaker terminals; it's a totally different chassis stamping. You wouldn't want to do that anyway because the MC275 is one of the most sought-after and collectible amplifiers ever made. When kept in pristine condition, used units can command nearly as much as new units. In some cases (early models, special editions, industrial versions), they can go for quite a bit more than list price. Tube rolling is where you can experiment, but be forewarned: That can be both addictive and expensive!
La Scalas are indeed very efficient speakers, so you won't need to be cranking things up too much with an MC275. That said, you will find the extra power provides greater control and more depth regardless of the music you're playing. Make sure you spend some time fooling around with speaker location; La Scalas move a lot of air and have big cabinets, so distance from walls and resonant surfaces will really change how things sound.
Have fun!
