Threshold 400A Amplifier and Conrad Jonson MF2500A

I'd be a little paranoid leaving the 400A on all of the time.  I had one and loved it.  The energy waste is huge so I only left it on if I was going to listen again in the near future. 
Why paranoid?  This is a fairly old unit and it well known that when they fail it can be catastrophic.  If they fry, it won't just be one transformer but most likely a whole bank of 8...
Use a surge protector if you aren't using a conditioner.  Also, have a tech check it out before leaving it on all the time.

No conditioner can be risky.  At least use some sort of surge protector or UPS.  I have a whole house surge protector (only trips on big hits).  Additionally, I have PS Audio & Blue Circle conditioners throughout the house.  We had lightning hit a transformer a few years back, it tripped the house surge, it totally fried one of my PS Audio pieces (Dectet), which in turn saved about $10k worth of gear plugged into it.
Would I go straight into an outlet???  Never, especially with something like a 400A which is amazing, impossible to replace and would be too expensive to repair if you do have a power surge.