And it's a sweet one, too.
A few years after I bought it I changed the tubes from the stock Chiinese ones to NOS Valvos. That improved the sound, but shortly after that when I was in San Diego I came across some Gold Aero Platinum Grade Gold Pin 7308s (exact replacement for the 6DJ8) for $35 each (a great find). The improvement over even the Valvos was nothing short of amazing.
If you want to improve your SP9 with only a relatively small cash outlay, I cannot recommend highly enough that you replace the cheap plastic ALPS volume control with a DACT stepped attenuator (24 steps, I think). The improvement is HUGE. You can get the DACT for a little under $200 from
www.partsconnexion.com. I think Michael Percy carries it as well. It's not a one-for-one replacement as you have to slightly increase the size of the mounting hole and run short wires from the DACT to the PCB.