When To Re-tube ARC REF?

What are the first audible signs of tubes that need to be replaced? I have a pair of ARC REF 210s that have had the factory mod to run KT120 tubes, which now have 2375 hours since I put them in. The hours suggest replacement, but I'm not hearing that yet, or so I believe. How would the sound first change? Thanks.
Non musical noise frequently heard when warming up is one indicator. Drop out, is another, simply non functioning lack of sound or low sound and the worst of all an electrical short or massive failure.
The spectacular shorting or "arcing" failure, which looks like a lightening storm in the tube. The Arcing failure can take out other elements in your amp , I had one such failure, an infant death no less and it melted a tube socket resistor and burned a wire. Another electrical failure is red plating but I don't know if that is related to age.
So if you think they are old you might want to take them out prophylactically. I see a lot of people decide to replace only when the tube won't hold the bias.
These tubes are shot replace ASAP you will hear more detail,more dynamics etc.
Time to replace the power tubes.

Average tube life will depend on several parameters: product type, how the product is installed, loudspeaker efficiency, room size and acoustic damping, listening habits or average sound pressure levels, A.C. line stability and purity, and other circumstances. Generally, preamplifier tubes last up to 4,000 hours, while power amplifier output tubes will last up to 2,000 hours. Near the end of their useful sonic life, aging vacuum tubes may degrade the sonic character of the product(s) they are used in. The sound may become somewhat dry and lifeless, with a noticeable decrease in harmonic richness or bloom. Bass response may be diminished, and musical dynamics may flatten out or compress. Toward the very end of their service life, tubes may become noisy, noticeable as a slight rushing sound or rustling noise. It is far better to replace vacuum tubes prior to the end of their service life, before severe sonic degradation or outright failure occur. Running vacuum tubes into failure may damage to other internal components and cause needless repair expense.