Is it common for tubes to outweigh the cost of the amp or preamp?

Viva tubes

https://www.vivatubes.com/search.php?search_query=kt88&section=product

Thanks for that link.

Yes even better on some of the equivalent brand, but if your can prove your in the industry and join/login I think you'll find "some" are better from New Sensor. 

Cheers George

go with the flow man.

I always wondered if Uncle Fester on the Aadams Family could light up a tube like he could with a light bulb by putting it in his mouth. I expect he could but the visual effect would not be as apparent. sigh

Ahhh....one of the beauties of owning tube gear is upgrading or rolling in new tubes. Enjoy the adventure!!

I own a variety of tubed audio equipment: AI Modulus 2B, Modulus 3A and Modulus 3B, BAT VK50, Juicy Music BlackBerry Extreme, Hurricane amps, a Response Audio-modded Ming Da preamp and personally hand-rebuilt Dyna
ST-70’s and Dyna MkIII mono’s. All have suffered the tube rolling endeavor. Lol

After some listening I am very happy with the purchase. From minute one, before any warming up or burning in, I noticed a significant improvement.

The new tubes have added so much detail to the highs and tightened up the low end which is resulting in a much bigger soundstage. They've given my HD820's a soundstage more comparable to how my HD800's were sounding with the stock tubes while still retaining the more full sound of the 820's. I'm also not experiencing the listening fatigue with my 820's like how I was with the stock tubes. I can listen to them for hours without a break now like I could before with my 800's.

Were they worth $640? For me, yes, and here's why. They are rated to last up to 15k hours so that comes to $0.04 per hour of listening. My turntable cartridge cost me $600 and is rated to last up to 2k hours which comes to $0.30 per hour of listening. So even if the tubes died after 2133 hours, that'd come to the same $0.30 per hour cost as my turntable cartridge. It was totally worth it for the improvement in sound I got when upgrading my cartridge. The improvement in sound from the upgraded tubes is just as significant and much cheaper when it comes to dollar per hour value. 

I look at tubes in the same light as my turntable cartridge. Better cartridges/tubes equate to better sound, and both wear out and need replacing so you're essentially paying by the hour with stuff like that. Considering it comes out to pennies on the dollar, I'd rather have a superior listening experience than pinch pennies and get subpar sound.