Mullard rectifier tubes are known for their longevity.
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I have an SLP-05 and I got a pricey tube (don't remember what off the top of my head) from Upscale. On all the time. Nearly 2 years and counting.
FWIW, I've shut the pre off a few times and it warms up in a heartbeat.
You have to balance the damage done by leaving it on with the damage done by powering down and up. Lightbulbs, by way of mediocre comparison, last far longer if you never turn them off.