Usually in tube amps (with output transformers) the smaller amps are the better sounding, usually on account of the output transformer, but also in how the power tubes are driven by the driver circuit (by far the lessor variable).
If you don't overload the amp I would see no reason to go for more power! It will just be wasted as heat, and highly unlikely that it would sound better. BTW the KT88s were likely more bright due to distortion rather than the tweeter (which is the same tweeter you were using with the EL34s). This is the same distortion that makes solid state amps bright- higher ordered harmonic distortion. A tiny amount more is audible as the ear uses them to sense sound pressure, so its very sensitive to their presence.
If you don't overload the amp I would see no reason to go for more power! It will just be wasted as heat, and highly unlikely that it would sound better. BTW the KT88s were likely more bright due to distortion rather than the tweeter (which is the same tweeter you were using with the EL34s). This is the same distortion that makes solid state amps bright- higher ordered harmonic distortion. A tiny amount more is audible as the ear uses them to sense sound pressure, so its very sensitive to their presence.