I am desperate to find a solution to this problem!

I have 2 Ubuntu machines in a home cinema, both are specific to their tasks. One runs XBMC and one runs MPD, they both use a simple ALSA setup to output audio over SPDIF.

Now the problem, I am about to change my amplifier. My current amp has a volume control, the new one does not (I need a preamp but can't get one just yet). At present the max I can turn the volume knob to is about 30%, beyond that the volume would get me noise pollution complaints. I am of course able to turn the mixer levels down but this does not seem like a 100% foolproof method. If the volume were to ever hit anywhere near 100% I can only imagine the damage that would be caused to my hifi and/or ears along with the relationship I have with all my neighbours!

So, what I need to do is be able to limit the volume of the digital outputs of the 2 machines to about 1/3rd so that if I were to go into alsamixer and put everything up to 100%, I'd still only be getting 33% of the maximum volume.

TL;DR - Is it possible to limit the output volume of SPDIF in a way that is almost impossible to accidentally reset without manually editing the configuration?