The 50 and 100 differ by about 3db wide open.The 100 (1959) has more bottom, more of a roar with the gain. They sound bigger and more open.
The 1987x has earlier breakup, more grind and is typically more aggressive sounding. Jeff Beck is a great example of classic 50 watt tones. Wired is a great example of albums to here this on.
Both of the RI amps are based on metal panel marshalls and not plexi circuits, I believe 1973 is the year they are closest to. They are brighter, have more aggressive gain, ect. I think both benefit from some minor tweaking to original specs, warmer bias and decent tubes. At the minimum clip the cap off of the bright channel volume and it will really help to warm the amp up. If it were me, I would by used because with the new handwired Marshall line I think the current PC Marshalls are priced a bit too high. You can find em used and clean in the 700-900 range. Great deals at that price point.
If you hit my link you will hear my 1993 1987x ( I just traded it) on most of the songs on my site. I had it tweaked to mostly 68 specs and it sounded great. An attenuator is a must with these, they will peel paint at 5.
