For this tube was used 500, 600 and 1k Ω, but in last years the market has settled on 600Ω, so 875Ω was in the mid of range. 500Ω will deliver more power and distortion/ harmonics.
Power 14W at 20Hz STM pretty good to a 90dB for a small room.
I should consider the core type and weight more important than the pri Z.
Please PM me with a price quote of a OPT like this: 600Ω, 30Wrms, C core, HiB 0.1mm, open frame.