Speakers are (2) 15" OB woofs and 1 horn per side, 8 ohms nominal.
Supposedly produce <39 Hz (never measured, but never feel I need a sub, music only).
Amp puts out 4.3 watts (2A3 tube) per side (tho mfr states can do burst peaks of 10 watts).
Medium sized room - never listen louder than 80-85 dB, peaks never get to 95dB.
Cable length will be 6'.
Currently running 9ga ! 
There probably isn't much if any downside to your current 9g cable, and I wouldn't go with much thinner than 12g because of the woofers. However, if you have modern woofers with more than a couple mm excursion, which you should running OB, then you should biamp. There are so many reasons why you could write a book on it, but it's not always easy to implement perfectly. If you get it right, it'll be better. I tried a bunch of woofer amps and honestly the cheap Crown Drivecore with built in xo is really good and super cheap. Your SET will also sound much better if it only has the horn to drive, and you could use a speaker cable as thin as 20g, but I'd go 17g. I'd also use solid core UPOCC silver, maybe 4 runs of 20g per side in a simple braid. It'll be very expensive but there's nothing better.
I have a 6W SET running a mid horn and supertweeter horn, 2x15" Acoustic Elegance 15" in a BR box driven with the Crown D-amp, I think the XLS... The horns use a really nice passive xo, bass uses the 24 dB xo built into the Crown. I built the exact same low-pass xo in passive and you really can't hear any difference vs the Crown's DSP xo.
