It is possible you are also running into limitations of the sound hardware, the SAM system is not only mono, but doesn't have much in the way of high quality sound reproduction, particularly the fact that the sound effects, music, voices, are all very lossy.

That being said, if you've tried the speaker upgrades the only other option I know of is an external subwoofer which will address any bass issues, and thus any power issues there. I think you can figure out how to implement an amp for the rest for the higher frequency effects, but I haven't heard of anyone doing that quite yet.

For Williams and DE/Sega games I've tapped the signal off of internal amps on the sound boards and cabled to the phono input of old receivers. And in one case (BSD) instead of a receiver I used a guitar amplifier.
Can't that be done with Stern machines?
Although the actual fidelity can only be so good. Garbage in, garbage out. But I have gobs of volume, tone control, and can put speakers where I want. And reverb/overdrive/saturation etc. type effects when I want to have fun with the Peavey.

I would suggest the awesome Pinsound board as this converts your setup to true stereo, has a full equalizer, allows you to mix in your own high-quality music AND keep or adjust the original call outs. Plus you can revert back to all original on the fly but with better audio... but they don't seem to do Stern's yet.

Bummer but it may come in the future... you could ask them if you're interested.

http://www.pinsound.org/

You could basically follow my acdc amp addition thread and buy a nice sub. The acdc thread applies entirely except the fact that acdc came with a nice enough sub to start so you'll have to buy something here.