imac low recording level from line level input

[[email protected]]

I just started using my (old = OS8.6) imac to record my music
"performances"
I connected the line out 1/4" jack at the back of my keyboard
amplifier and connected that to the audio in port of my imac.

I then downloaded opcode's vision software and used that to record the
audio. It worked moderately OK except that the volume is very low
(i.e. very low recording levels). I also tried using the line out from
a different amplifier (bass guitar amp) and got the same results:
basically OK but rather low levels.

Should this setup work OK? I presume the amplifiers are basically
sending out a line level signal but I need to turn up the volume on
the amps rather high to get anywhere near a significant output.

Would things work any better if I connected my amp(s) to a small mixer
like the Behringer UB502 and sent the output from that to the Mac or
would that not give any boost to the audio signal?

Or should I be doing something on the imac itself?

Thanks in advance,

Henry