I own:
EVH 5150 III 50w 6L6
EVH 5150 III 100w Stealth
EVH 5150 III 100w EL34
I don't have an OG 5153 100w to compare to the Stealth 100w amps so I can't really say what's different there.
As for the 100w Stealth and 100w EL34, in my experience, the Blue and Red channels of these amps are
completely identical. The poweramp is identical except that obviously one is tuned for and uses 6L6's, and the other uses EL34's. My 100w EL34 version to my ear has slightly more prominent mids than my 100w Stealth, but only
sliiightly, and those extra mids can be almost exactly accounted for by lowering the Mid knob just a bit, and exactly accounted for with any capable Parametric EQ in post.
As for the EVH 50w amps, the 50w EL34's Blue channel has less low end filtering at the input than the 50w 6L6's Blue channel. This can be remedied by clipping one leg of the C137 component on the 50w EL34's board. Other than that and the power tube types, those amps are identical. However the EVH 50 watt amps are quite different than the EVH 100w amps. Mostly, the 50w amps are slightly more "slinky" sounding and feeling than the 100w amps. I can almost get my 50w 6L6 head to have the same response as my 100w amps by using a mid boost at the 50w amp's input, if that tells you anything. Also, the 50w amps don't really have more or less prominent low end than the 100w amps, but the sub lows of the 50w amps don't extend as far down the spectrum as the 100w amps.
Here's another interesting anecdote about the 50w heads. In my experience, EVH amps tend to vary in sound from one to the next. I'm not talking about different models, but two instances of the same model. This is nothing unique to EVH, a lot of amps from a lot of amp companies are like this, but I've tested EVH amps myself and it's fairly significant. When the Stealth 50w was announced, I preordered one. I already had a 50w 6L6 head and wanted the "updated" Stealth 6L6 version. I got an email from my preorder dealer right as they released which told me my preorder was going to take "a couple more months," so I ordered another one, intending to cancel the original in the next couple days. However, about 2 days later, I found out that the "couple months" was a bit of an overstatement as both amps arrived at my address on the same day. So, at that time, I had a 50w 6L6 and two 50w Stealth amps in my studio at the same time, and about two weeks before the return period on the first amp was up. So, I did some testing.
To sum it up, one of the 50w Stealth amps sounded basically identical in character but overall slightly thinner in the lows and low mids than my 50w 6L6, with no knobs I could adjust to change that general character, and the other 50w Stealth was basically identical but slightly thicker sounding than my 50w 6L6 in the lows and low mids. Funny enough, I ended up keeping the "Goldilocks" amp, my original 50w 6L6 head, and I returned both Stealth 50w amps. Sound wise, as far as I could hear, if you account for the individual variances of the two 50w Stealths and took basically the "average" of the two, then it seemed the 50w Stealth and 50w 6L6 circuits were pretty much identical. The Blue channels of the 50w Stealth and 50w 6L6 were the same. Same character, same amount of Gain (lots of people say the 50w Stealth's Blue channel has more gain than the 50w 6L6 Blue channel and I can assure you this is not the case), same everything. Same deal with the Red channel. They're the same on both amps. As far as I could tell, besides the 50w Stealth having external bias points, the biggest differences between them was the color. In that sense, we've kind of already had a model of the 50w Stealth's Blue channel in the Axe even before this latest update.
