The RKV is in general bad with low impedance headphones, and adding an impedancer goes against the OTL philosphy of the amp. I briefly tried W5000 (40-ohm) with RKV, and it was not a good match. Too much noise to hear through.
When I first got the RKV, I was surprised by how fuzzy it sounded with K1000. The combo was musical and enjoyable despite its shortcomings. However, swapping the (archaic) stock opamp for OPA637 addresses many of these shortcomings. I really like K1000 + RKV + OPA637.