What kind of speakers are you using? Also, how is the room furnished and how much soft surface is there? Too much thick carpet, stuffed furniture, and especially heavy drapes and wall hangings can overdamp a room, killing the treble unless you are directly in the sweet spot.
Also, 12AX7s would definitely brighten up the sound compared to 5751s. Also, better power cords won't specifically add treble, but the right ones make everything more clear and dynamic, but the clarity applies to all frequencies.
Also, 12AX7s would definitely brighten up the sound compared to 5751s. Also, better power cords won't specifically add treble, but the right ones make everything more clear and dynamic, but the clarity applies to all frequencies.