Wow...this has gotten a bit deep.
Here's the thing on using resistors for EQ. It doesn't work that way. Yes, adding a resistor in series with a SS amp output may result in a change in response, but that change depends on the load impedance curve, which when combined with the resistor creates a frequency dependent voltage divider. If the load Z were a flat curve, there'd be no change in response, no "eq" to be had.
The Zobel network Wiki article...um... that's not using just resistors for EQ, there's always a reactive element. You can't EQ anything with just resistors, no matter how many there are. Funny that article was referenced, I've used those things. I can promise you, there's way more than resistors in there!
As to the sound of tubes (deep breath), there are several simple reasons for the "tube sound". One is that the plate impedance of the typical output tube is too high to drive much of anything, so a transformer is used. The output impedance of the combination is lower, but still not as low as a SS amp can be. Transformers can be shockingly good, or not so much. The quality of the transformers relates heavily to price and size. The combination of output tubes, biasing, output transformer design, where you get your negative feedback from and how much, all of that is a study in compromise, with the goal being flat response, low distortion and adequate power. It does take a bit of deft engineering, and high quality (expensive) parts to make a tube stage behave really well. And so...when compromises are made, we end up with outputs with somewhat higher impedance, somewhat higher distortion, and somewhat lower power. Drive that into a reactive load like a speaker or some headphones and you now have a non-flat response along with those other characteristics. And yes, it does sound and measure differently. But it's not really fair to say all tube amps sound the same any more than saying all speakers have a flat impedance curve. None of that is true, so the end results depend on the specific amp and load. There are some fine points to the tube sound, but that's most of it.
By contrast, solid state devices are by nature lower impedance, higher current and lower voltage devices. That means they can operate directly at the desired output voltages and power while providing a low source impedance. Designing all of that is simpler, cheaper, and easier get better results from. And that's primarily why people recognize a difference in tube amp sound.
Can you make a tube amp sound like a good SS amp? Sure, but it's not done all that often. A lot of tube designers wouldn't want to, they like being different. And yes, some good tube amps are indistinguishable from good SS amps.
Can you make a SS amp sound like a tube amp? Sure you can, and it's been done. And a large part of making that happen was adding an output resistor to simulate higher output impedance, though, as I said before, the final "EQ" happens not just because of the resistor, it's the load too. But, and this is just an opinion, the reason that these ideas don't succeed has to do with the fact that there aren't any tubes to look at, see glow, feel the warmth, etc. Big tube amps fill several sensory inputs, not just ears. And all of that input has an undeniable effect on perception.
There's also a historical reason for some people's preference to tube sound. In the early days of solid state, there were some really bad designs. Some of them were really awful, noisy, dull sounding things, mostly because engineers hadn't figured SS out yet. It didn't take long for those days to end, but during that time tube amps did sound much better. I remember owning tube gear, then building a solid state amp and wondering why it didn't sound nearly as good. But it wasn't magic, test gear showed very poor performance from the SS amp. Anyway, the idea that tubes are better got some pretty deep roots pretty early on, and it's been hard to shake.
I once had a chance to run a bunch of tests on a Marantz 8B tube amp. It was pretty impressive driving a load resistor, but then I drove it into a complex load. I wish I still had the response plots, but they were NOT flat, and quite dependent on the load. Don't know why I mention that without the pictures...but oh well, it was a long time ago.
