It's treble control has a wider range than regular BF amps, as it can actually cut treble. http://bmamps.com/Schematics/fender/...lux_manual.pdf
If it's still too trebly, maybe there's something wrong with the amp, eg bad tube, bias too cold?
Has it always been this way?
If so, then yes, a different speaker type may significantly change the tone, but there's no need to go to the hassle of changing it from 2x10 to 1x15.