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

you can't go wrong with vintage tubes from the US or from Europe; even 80's era East European tubes are far superior to what is coming from China and Russia, which is what you find typically in even high end gear. Maybe the best way to describe vintage tubes is to look upon it like cooking; an old family recipe or institutional fare. 

One way to save a few $$ is to buy ugly tubes and pairs and quads that were all manufactured the same but we're labeled for various companies.Sometimes a couple of new production tubes alongside of a couple of NOS sounds good too.

I don’t know about that but I did spend more on Herbies Tube Dampers than I did on tubes. 

samiamnot-

No insult intended with my post. 

The intention was to raise awareness  that perhaps one might consider  starting with a substantial "box" before additional expense-especially if it exceeds the value of the actual box.

I use tubes in my system-amp,tuner and phono. Using NOS tubes that cost more than the equipment, won't ensure anything. Since it's a headphone amp, I would be more concerned using QUIET tubes. 

If you do find it improves your experience, that's all that matters. It's all subjective, YMMV and all that.

Considering tubes that cost more than the equipment, would be a reason to reevaluate my original purchase, and perhaps upgrade to a better unit.