Speaker Cable Recommendation

MOGAMI.  Excellent bass.

You might be able to get away with Nordost blue heaven speakers cable, because of your tube amp and preamp.

Get Auditorium23, specifically designed for high efficiency, SET application and the next best thing to buying $$$$ Shindo originals

Consider vintage Western Electric cables.

There’s a seller here.

Have you rolled those 12AU7 s?

"Not a great room". It seems counter intuitive to me that you would want to spend up to $1000 to counterbalance the room problems, especially when expensive cables tend to be neutral and likely to expose problems in you room/system. In your place, personally speaking, I’d cheap out and get some Belden 10 ga cables sold by Blue Jeans. They might just do what you want, and if not you won’t have spent much money. The Mogami (12ga) is a good option too, but it's different from the Belden in that it is more info rich in the mid-range, but not high on warmth.