At this point I compared the Blackstar against the Katana and still preferred the tube amp. I tweaked the EQ by ear and that got me to a better place, with a little more low end and a little less highs.
What you are missing here is the fact that the "magic" of a tube amp can't necessarily be properly measured by a mere frequency response curve, because you lack any sort of phase relationship data.
The "magic" of a tube amp is the interaction between the tube(s), the output transformer, and the loudspeaker driver(s). It's not the tubes, it's the whole circuit topology, and the complex interactions of the relative and characteristic impedances that are inherent in the fact of a high output impedance source being transformer coupled to a very low impedance transducer.
This article will help explain why.
Why do Tube Amps and Solid-State Amps Sound Different?
www.electronicdesign.com
In the debate over tube amps vs. solid-state amps, Part 1 discussed the differences in applications. Part 2 concentrates on the difference between transistors—MOSFETs and bipolar...
www.electronicdesign.com
I was doing some testing with some new overdrive pedals some years ago, and running them into my Ampeg VH-140C combo amp (solid-state). Everything just sounded somehow a little "two-dimensional". Like, it sounded good, but like some indefinable thing was missing. And then I plugged the exact same signal chain into my Mesa/Boogie F-50 (tubes), and everything was suddenly *three-dimensional*. Like, it just somehow *came alive*.
Solid-state amplifiers definitely have their place, and they are very "accurate", but that's all they will ever be. Tube amps aren't accurate, but that's why we love them for making music.
As has been famously said, to paraphrase, if you can't measure it, and you can't hear it, it doesn't exist; if you can't measure it, but you can hear it, you're measuring the wrong thing.