[CamB]

does a stereo amplifier draw any current when the ignition is off?

I don't think they are supposed too! Having said that, my old car (with three amps) would flatten its battery when not used for an extended period. It only happened after the stereo was installed.

i noticed the installer spliced the constant 12V on (yellow) wire to one of the big amp power supply wires too.

The constant 12v on is usually on the head unit and is to allow it to retain its memory. If this is the wire you mean, it should be ok for it to be spliced onto the amp's main feed. If this feed is downstream of the fuse, I would expect a minor current draw (for the head unit's memory).

Are you certain the head unit's accessory wire is being used to turn the amps on (ie, that they are not on all the time?).

If you want a quick fix while you are away, you could just pull those fuses and leave them out.