Semi, you had disappointing results but you don't say anything about the transformer setup you were using. What was its power rating ( in VA, which are more or less equivalent to watts ), and its make ? How much power did your amp draw ?
Malickia, Pauly suggests you use a tranny 10 times too big. I wouldn't go quite so far--I would be comfortable with 3 times oversize and I would be willing to try 2 times--but 10 times oversize ( heavy ! ) would certainly do no harm. The idea is to make sure the transformer's iron core will never saturate and limit current.
The best transformers use oversize cores anyway; my two Topaz ultra-isolators weigh twice as much ( for their power rating ) as the others I use in my second system.
For more info about iso transformers, look for posts in the archives by Sean and Lak.