You can't put el84s in a 6L6 amp, the sockets and plate voltages are totally incompatible. Amps that are designed for el84s usually use about 300-350 volts dc, whereas a 6l6 amp will use 450-500, which would destroy any el84 tube.
You might be able to use el84s with a special ''yellow jacket'' adapter, but they will overdrive at much lower volumes and I can't see that giving you a ''nice clean sound'', more like a nasty, dirty sound. Maybe you mean el34's - they can sometimes be directly substituted.
The 6L6 is known mainly as a ''clean'' tube - that's why fender has always used them. el84s or el34s would only make your amp dirtier-sounding.
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