First, some recriminations...
The first rule of fixing a solid state amp is don't connect a load (speaker) until you know there is no DC on the output. If you don't follow this rule you stand a good chance of blowing up an expensive speaker. It sounds like the original speaker is toast and if you had the test one on for more than a second or so you may have damaged that too.
It would have been a big help if you have mentioned the manufacturer, Roland, in the title and polite to include the schematics so we don't have to go looking for something you already have.
NOTE:I've edited this next paragraph to avoid future confusion as I was looking as the wrong schematic....
Start with the power amp on page 7. It's a simple one. First measure the DCV across the output (no speakers!) to confirm what I think we already know. Assuming you find more than 0.1V or so we can proceed to testing the output & driver transistors Q7, Q17, Q9, Q16. I prefer to remove them to test in case one has failed open. Also check the and resistors R34, R46, R30, R49, R33 and R45.
Report back and we'll go from there.