Leaving tube amps on.

You really are best taking your advice from the Manufacturer.

Why!!!

When the worse that can happen, does happen, the Home Insurers' will be gathering their info to decide on Payout from that very place.

Output Valve Red Plating is able to occur in an instance.

The following is a Guidance from an individual much more experienced than myself:

Red plating is always a BAD sign and if you ever see it, turn your amplifier off immediately.

If a valve suffers from this overheating for more than a minute or so you should probably consider it written off.

I suggest being present as much of the time the Valves are in service, as this will enable you to turn the Valves of in an Instance.

Those beautiful glowing tubes are not visible for the ambience they can create only.

They are visible to allow heat to not collect in a case, and enable eyes on the prize, when it comes to witnessing changes in their operation that can cause a fire.

I was first introduced to Valve Equipment in the early/mid' 90's and was quick to move over to the schematics using Valves in place of Transistors and Semi Conductors.

I had my first Valve Amp' Pre and Power commission built, and the EE who at this time had a good few years of working with Valves to produce Audio and Studio Equipment gave me as a newbie a clear warning.

Valve Equipment is not a Turn On and Forget device.

Valves in use need monitoring during use and after Power Off.

I have kept to my side of the Bargain for the best part of 30 Years and see no need to make a change. The same EE remains a friend and has produced further Power Amp commission builds for me. 

Red Plate is common on a Output Valve and is a result of incorrect Bias, where increased Voltage is enabled.

Everything in Valve Talk is about reliability, maintained Bias is extremely important.    

@immatthewj No need to give a in depth explanation.

It was a reliability issue

My EE will keep a Tube Designed Device under scrutiny for weeks to ensure the circuit is not compromised.

Even when doing general repairs on any Tube Amp' the device is kept back to thoroughly check for reliability. This approach has lost lots of customers who want the device to be sent for servicing / repair returned within a week of having sent it away. 

I recall back in the 90's when Tubes Amp's were becoming quite popular, it was EE's like mine that were the resource selected to repair certain Brands Amp's as the producer did not know how to trouble shoot their faulty circuits.

I know my EE during the 90's has trouble shot a few Brands faulty circuits as a result of receiving Amp's that had multiple failures, usually as a result of extended periods of usage.  I also know some of the circuits worked on were fire risks, due to how the Output Valve / Tranxs could be affected. Maybe he has even more of these circuit corrections under his belt in the 00's ? I have not inquired for a long time.