I did "splurge" on a $200 NOS Mullard ECC32, but that was just because I felt like it. I used the $50 RCA VT-231 for months, and it sounds awesome. The ECC32 is only for the tube-obsessed. There is no need to go to that length to get awesome sound from the SP Extreme.
Again, except for the few overpriced 6SN7 variants, the tubes for the 337 are not cheaper than the SP Extreme. They use the same power tubes, and since the 337 requires 2 driver tubes versus the single in the SP Extreme, it's really a wash.
The only price difference is in the amp itself, which with shipping, comes out to about $250-300. I think that makes the 337 the better value than the SP Extreme, but I do think the Extreme is a little better sounding.
Also, with adapters, there are a ton of tube rolling options with the SP Extreme. The 337 does allow plenty of tube rolling possibilities, but not the same amount that the Extreme does.
FWIW, I still own both and am keeping both. The Extreme lives in my office, and the 337 lives in my family room.