What about Vac Sigma?
"Tube compliment: [from manual]
3 x 12AX7 (phono option) (located on circuit board at left side of chassis)
2 x 12AU7/6189A (line stage)
2 x 12AX7 (line stage)"
Regulated PS, pretty similar to your SP8. I don't know if the change would be worth the hassle. VAC makes some nice stuff.
You are DIYer, so what about a kit? Aikido Octal is a solid choice. It sounds great and has an easy to assemble kit. With some excellent tubes you could build it for <$1000. It would be better than SP8. You would lose half of your parts investment and all of your labor investment if you sell it, but that is still cheap. The sound is VERY good. You could put an aikido phono stage in the same box. There are other preamp designs worthy of building.
Another option is Van alstine T9. They have been sounding MUCH better in the last few years. The AVA sound used to be fast and clean with minimal soul. But maybe the new T9 with 6CG7s has a little more body. I had a T8 with russian tubes, it was midfi sounding smiley face EQ. So you have to listen before you buy. They have refund policy but you have to battle with Frank on the phone. I heard the newer stuff a couple years ago it was excellent. Incredible value. SS opamp phono stage, remote, new <$2000.