Hi there,the wattage ratings on trans. refers to the amount of power the trans. can handle from the amp.the trans. doesn't make power,so a 300 watt tranny will work fine with 150 watts feeding it,there's no self regulating going on,the trans. is simply working at 50% capacity.As far as SVT tranny's go,the really old ones,(from the era of the hard wired 4-pin XLR spkr. conn.) had 2 seperate 150 watt taps,on for the 4 pin XLR and one for the ext. spkr.,the new ones have a 300 watt winding.If you could find an old one you might be able to make it fit,rather large though,good luck Mel.