Tube preamplifier lifespan

With most tube gear, you can expect a MUCH longer serviceable life than solid state gear.  Most parts can be replaced on tube equipment while many solid state gear becomes unrepairable because a chip or transistor is no longer available.  Also, tube gear has far less parts because the circuit is simple.  One of the technicians at a local shop restored a solid state receiver as a labor of love.  It took, among other things, replacing more than 200 capacitors; this would never be done as a commercial repair.  
 

My tube preamp is about 15 years old and has required no repairs nor replacement of any tubes.  Moreover, most of the parts were very old when it was built by a custom builder.  The power supply is new, but the audio circuit parts are up to 80 years old.

I'm listening to a 25 year old preamp it sounds great and works great haven't had a problem with it besides an occasionally dirty pot and changing tubes. It's a custom job so kind of a one off but tube preamps can last very long and plenty have. 

I have a Sonic Frontiers preamp built in '94 that still sounds great.   Many small signal tubes can last 10,000 hours , that's a lot of listening time.   

I've had it about 5 years and it still works flawlessly.  

This issue just came up with me this month.  I was interested in “daisy chaining” the balanced output from my McIntosh preamp output to my SVS subwoofer input, then balanced output from the SVS to a PS Audio BHK 300 balanced input.  That arrangement would save me significant new in-wall and attic install of a balanced cable run.  The SVS tech was concerned about combining the two components, and specifically the potential additional circuitry strain on the output of my tube preamp output.  The exact point being, it was because I was using a tube preamp.  He said many tube preamps might not be able to handle it (he looked up specs of both pieces of gear and started crunching ohms).  There wasn’t published the necessary spec for the McIntosh preamp, so to be safe, he encouraged me to contact McIntosh.  The McIntosh tech was a bit put off by my question, discussing potential parts wear, saying “we build McIntosh preamps to last 30 years.”

My Lazarus Cascade Basic preamp was bought new ~ 1987 and is still going strong.  I've replaced the power supply caps and resistors, along with some audio caps.  The metal film resistors are all good.  I suppose it depends on a particular preamp, but no issues with this one.

https://photos.app.goo.gl/xNJNjZzSi8vAq67V9