Fender Bassman 100 combo woes

[DarkHeart]

Hi all, im new round here, i just bought a Fender Bassman 100 combo on fleabay and for some reason I expected it to be better than my Behringer 1200 and well basically it isnt, id read reviews that were very favourable each one commenting on the sound quality which to an extent is true but they also said it has a 15" speaker, only now do i find the reviews of the amp ive bought with a 10 inch speaker which isnt very loud! My question is, when i plug headphones in i can hear both through the phones and also the onboard speaker, is this right? can i just unplug the speaker so i can use it silently or have i bought a pup? Also, if i bought a seperate cab and plugged it into the back does that cut out the internal speaker and would it be worth investing in.