AMP Wattage!
Please understand one thing. Most amps are rated for their advertised RMS wattage at clean signals. On most amps that's going to be at 3-5 on the volume knob. So your 100 watt Marshall puts out 100 watts at 4. You like to play it at 7, and if your amp is healthy, that's when it puts out 150-180 watts (or more). The guys at Marshall knew what they were doing when they put the "100" logo in the corner of the cabs, but that still meant that you needed two of them to handle a 100 watt amp that put out 180 watts at the levels you set the amp! Get your amp tested for it's real output wattage, no one likes the smell of a melted speaker, OK? Always figure you need twice your RMS wattage in speaker power handling to safely run your amp on 10.
If you get two 30 watt speakers to run with your 50 watt amp, and dime it, they'll be good to go for awhile. I don't know what timeframe "awhile" is. I've safely done it for 2 hours straight, without issues. I have not done that for extended periods, so you want to take that into consideration with all the OD pedals, clean boosts, and so forth that push speakers to raise your level for solos. Use your good old common sense here. Sure, your old small block sounded great and was running awesome at 8000 rpm for "awhile", then the crank, piston, or connecting rod broke, and you weren't mobile anymore. Use the same common sense judgment with choosing your speaker wattage.
