Perfectly fine to leave it on 24/7 according to MF, it was designed to be left powered up all the time.
I would tend to only leave it powered up 24/7/365 if your ambient room temperature is pretty "European" (ie: 68F) if you live in the tropics and don't have air conditioning it may be a good idea to power it down quite regularly or invest in some air conditioning.
The "scorching" around transistors has been mentioned by a few people but I think this is something that is pretty rare and happens to only a very small percentage of X-CANS. The 10ohm resistor value plays a pretty big role in temperature and if those 10ohm resistors turn out to be a couple of ohms less then this increases the heat output.... most of the "scorched" boards I have seen have all had 10R resistors that measure more like 8R. Plus, the resistor power rating is most important here.
You could put two X-CANS side by side in the same room, one will run as cool as a cucumber where the other will run warm, luck of the draw and neither is wrong or right.... there are ways and means, other than the resistor / heatsink mods of cooling an X-CAN or there are other things you can do to actually "increase" the heat whilst, at the same time, fitting components that will withstand the increase in temperature. Remember, it's "not" the transistors that scorch the board it's the
resistors overheating so..........
Generally, with stock valves (JAN Philips 6922) you should be ok to leave them powered up all the time.
By the way......
these are pretty good
EDIT: On the V3 MF now "raise" the resistors that are likely to go smokeside up off the board to A: allow them to cool a bit and B: to protect the board should they burn out. OK, that's "one" way and pretty much a string and sellotape fix instead of just fitting higher powered resistors that will withstand a nuclear attack, these manufacturers work on penny margins though where an extra few pence on a better part would add pounds to the RRP.
Musical Fidelity's Raised resistors "in house" workaround mod:
