If you're using a JBL E120, it will blow before it breaks up sonically, or your ears will break up. The E120 is all about clean uncolored sound.
Other speakers -- Celestions, etc. -- are valued because they break up and sound dirty at low wattage, this gives certain amps their tone characteristics.
There's a big difference between tube power and solid state power, as well. My 85 watt Twin Reverb tube section was much, much louder than a 200 watt solid state power amp driving my E120s.
For a solid state power amp, I would get a bigger one and, obviously, not turn it up too high.
For a tube power amp, where you are looking to get a tube-saturated sound with the attendant compression and overdrive, smaller is better ... which is why lots of guitar tube power amps can be run at half power or full power, so you don't split your ear drums trying to get the tone you want. A Fender Deluxe Reverb is 22 watts ... and to get it to sound great, you turn it up to 10. Try that with a Twin Reverb and you'll damage your hearing.
Note that the McIntosh tube amps are not guitar power amps, they are audio power amps and they are not designed to add distortion to the sound. Jerry used a solid state amp so it would not color the guitar sound, he wanted headroom -- read lots of clean power with zero distortion.
A 1000 watt power amp -- Weir's Godzilla, for example -- will give you miles of headroom that you'll never use.