Yes and yes, in a way. You will definately get more bottom especially if your heads are more than 10 watts. I find that it doesn't take a lot of wattage to push most speakers, even higher ratings. But that all depends of course. If I were to push my Maz 18 through a 4x12 with EV12Ls (200 watts a piece) it wouldn't make much sense. And the volume will not increase but the perceived volume will because there is more surface area generating the sound and you will move a lot more air that way. Hope this helps!
-Justin
'67 and '74 Fender Twin Reverbs, '74 Marshall 1987 lead mkII, Metro Superlead 100. Pedals from TC Electronic, Ibanez, Dunlop, BK Butler, Electro-Harmonix, Fulltone, Maestro/Gibson, Loopmaster switching, VoodooLab, Boss. Gibson and Fender guitars, Dimarzio pickups.