While @imhififan brings up a very probable cause, I can assure you that the BHK300’s don’t lack treble, or some other frequency anomaly.
I’m using them with a very revealing speaker, the TAD CR1. I’ve never heard the speakers sound better, even against TAD’s own electronics (though that was from models ago)
Just curious why you got rid of the BHK’s? Especially with some tube rolling, they are an amp I don’t believe I’ll ever get rid of, and know of one person who thought they sounded better than his Constellation amps, which were also designed in part by Bascom King, but are fully solid state. Perhaps someday an amazing GAN amp with tube input stage may better them.