So what makes a Fender Blackface Deluxe Reverb, Bassman, Super Reverb, Twin Reverb etc. sound clean and sparkly?  There are some common design elements that make a clean amp clean?  Why do clean amps rarely seem to overdrive well? I'll try to answer these questions here.

What are some common design elements make a clean amp sound clean?

1. Fixed Bias---most really clean amps are fixed bias. This gives an amp a firmer bottom end and more clean headroom.
2. Higher Plate Voltages: Higher voltages on the plates of the power tubes reduce the midrange and breakup latter than lower plate voltages.  For example a Fender Twin Reverb (the prototypical clean amp) runs it's 6L6GC's at ~460VDC while the Fender 5D4 Tweed Super (lots of bluesy breakupand midrange) only runs it's 6L6GC's at 360 VDC.
3. Tone Stack positioned early in the circuit that scoops out midrange--- this is common to pretty much all the  Blackface Fenders.  By positioning the tone controls early in the circuit you have some signal loss. The classic Fender tone stack naturally scoops out some mid-range.  More mids equals more breakup so losing some mids helps keep the amp clean.
4. Reduced coupling caps value into the phase inverter---this cap is all important in scooping out midrange.  Blackface and Blonde Fender often use .001 uf.  Tweed Fenders and Marshall's use .022 uf that's 22X the capacitance.
5. Cleaner speakers---many times but not always a clean amp will be paired with speakers that simply breakup less or have less midrange. Hence the sound is cleaner.

So why do clean amps often not sound a thick and gritty when pushed?

Clean amps often do not breakup as nicely when push because a lack of mid range. This explains the often biting treble you hear when you turn certain amps up.  Also higher plate voltages make the power tube distortion sound harder.