KT150s in a Preamp Design?

You can use big power tubes in a pre-amp. Remember a preamp is just a low power control amplifier. It's a bit of overkill because a preamp doesn't need to make the kind of power a large power tube can produce. No idea on the sound quality though. 

Yes you can, in power supplies, ARC for example is using 6550 in some of their Ref line. So why not a KT150.

Got it.  That makes sense.  I'm wondering what the sonic benefit would be.  Seems that 6SN7 would be big enough.  A KT150? My lord!

My phono preamp uses 7581A 35w power tubes in the power supply. The big supply gives more depth, space and air to the soundstage.

I have a preamp with a separate power supply made by the company you posted the link to.  Noromance is right, my power supply can use two 866a Mercury tubes it made a big difference from the stock 274b tube that came in the preamp. The bigger tubes also make a big difference in the amplification stage in the same way, even in a preamp. The guy who builds these has excellent feedback on cunuck audio mart.