the 9V bit is moderately important - anywhere between 8 and 10V DC should be OK. From memory they're negative to centre, but do check it to make sure. They may have changed at some point!
As for current, you'll probably need at least 500mA and possibly as much as 1A. As a general rule of thumb it should at least power up with 500mA but a higher current is perfectly okay (and indeed desirable, for larger instruments with very large speaker systems).
