Hi, and welcome to the Krell family! I’ve got a FPB-300, and it runs warm even in standby. So I turn it off when not in use. I’ve heard others say they hear a difference between a cold start and a warm start, saying it sounds better after an hour or more of warmup. But I have not experienced this myself. It sounds pretty much the same after only 15 minutes of warming up. I watch mine with one of those thermometer pistols, and it reaches full temp within a few minutes of being turned on, so if it’s running 105 degrees, wouldn’t it sound the same as if it has been left on at 105 degrees? But to each his own, it can be difficult to reach the back depending on where you place the amp. But I’d watch the electric bill, it may cost you $30 to $50 a month to let it run. Depending on how often you use it.
Personally I’m not seeing any real benefits to leaving it on. It sounds the same to me after only 10 to 15 minutes, if that. And isn’t bad when first powered on anyway. I feel better knowing it’s off, saving wear and tear on the caps. And don’t feel like I’m loosing anything in SQ having it off. Enjoy that 600! That’s a real amp!