I own a 50w 6L6 (version 2, with the concentric Gain/Volume pots on the green and blue channels) and had two 50w Stealth's at my place at the same time for about a week to test them out.
The 50w 6L6 and 50w Stealth are almost the exact same amp. Under a microscope, you can hear that the 50w 6L6's Blue channel is sliiiiiiiiiiiiightly less compressed under some Gain settings, but other that that I found them almost exactly the same. Any real differences between them could probably be attributed to differences in component manufacturing tolerances, meaning the difference between any two 50w 6L6 amps is going to be about as pronounced as the difference between any given 50w 6L6 and 50w Stealth.
The 50w EL34 is slightly different. The EL34 power section means there's more damping in the power section, which means the mids are just a tad bit more prominent. The 50w EL34's Red channel is the same circuit as the 50w 6L6 and 50w Stealth's. The 50w EL34's Blue channel doesn't filter out as much low end as the 50w 6L6's Blue channel, so it's a bit looser and more woofy, but like you said, the difference can be minimized with the C137 mod.
I also have a 100w EL34 and the major difference between the 100w and 50w amps is that the 100w amps are slightly thicker or fatter in the mids, not in the output EQ but in the preamp. The 50w amps can be made to sound slightly closer to the 100w amps by slightly mid-boosting the 50w's input) and the 100w amps extend farther down into the low frequencies than the 50w amps.
