KT150s in a Preamp Design?

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.

Hey Noromance, what is the make/model of that phono stage?  Sounds interesting.