If you go with a tube dac because you're trying to fix something like brightness in your system, it almost never works. You first have to determine what's causing the problem. It could turn out to be the amp, preamp or speakers. Its best to deal with the problem component directly, than it is to buy other components to offset the short comings of the offender. Even if you do manage to get things right, upgrading is a nightmare.
A different approach would be to find components that are better at creating the correct timbre of the instruments on the recording. Because that's the real issue here. If the highs are bothering you, you have 3 basic choices. 1. Do nothing and suffer with it. 2. Try to cover up the problem with products like tubes and cables. 3. Eliminate the actual problem. Option 3 is the way to go.
A different approach would be to find components that are better at creating the correct timbre of the instruments on the recording. Because that's the real issue here. If the highs are bothering you, you have 3 basic choices. 1. Do nothing and suffer with it. 2. Try to cover up the problem with products like tubes and cables. 3. Eliminate the actual problem. Option 3 is the way to go.