The Alps "Blue Velvet" RK27 is almost ubiquitous in analog preamps and headphone amps below $10K - especially tube units which are less likely to use a digital volume chip. Which is a shame, because EVERY TIME I’ve compared this pot to a better alternative - RK50, TKD (the 2CP-2511 is fantastic!), stepped or ladder attenuators - the RK27 has been exposed as a significant performance bottleneck. JUST like you’ve found. Alps used to make an RK40 "Black Beauty" pot which was better, and much more reasonably priced than RK50, but sadly it’s long discontinued :(
Now every time I see a component with that dark blue plastic Alps pot inside, it’s enought to make me lose interest. If it's also above $10K MSRP, I cringe. I think it’s become so common for its low relative cost and motorized option (and laziness on the designer’s part).
I’m pretty sure a digital volume chip sounds better than RK27 too, but I haven’t compared them on "relatively equal" terms - just guessing here.