In this post:
http://pinside.com/pinball/forum/topic/psw10-amp-only-mod-for-tron-le
I detailed how I removed the amplifier section of a PSW10 and added it INTERNALLY to my Tron LE and connected it to the Flipper Fidelity subwoofer I had already installed in the cabinet. That works very well, but costs about $70-90 depending on when you buy the sub.
If you want a cheaper alternative that doesn't cannibalize a PSW10, you can use a car amplifier. I purchased this one:
http://www.amazon.com/Boss-Audio-R1002-2-Channel-Amplifier/dp/B004S50ZB2/ref=sr_1_2
It's $29 shipped if you have Prime. All I did to connect it was make a power cable to connect to the Stern 3 wire accessory plug (remember you have to connect the +12V to both the power input *and* the "remote" input to cause the amplifier to turn on). Then unplug the subwoofer and make a cable that will connect to the wires you unplugged and go to a single RCA jack. I tried going to the speaker level inputs on the car amp (that's what I used on the PSW10), but it was expecting a much higher level there, apparently. But the RCA plug worked. Then make a cable to connect from the speaker output on the amplifier and go to the speaker in your cabinet.
You will still want to tweak the "input level" knob on the amplifier to taste. You may want to make your cables all long enough you can move the amp to the coin box area to make setting the input level easier, because I found that a setting I liked with the cabinet and coin door open was a tad too much once I put the glass back on and closed the door. Made things rattle. But turning it down just below the rattle level was a big improvement over having no amp on the sub.
Yes, I know, it won't sound "as good" as a PSW-10 sitting on your floor. If you don't mind a PSW-10 on the floor and want the best sound possible, DO THAT. If, however, you don't want an external subwoofer, this is an easy and cheap mod that will get you much better bass out of your pin.
Another note: I would be careful about what car amp you choose. Some are power hogs and may suck too much current from your game power supply. I tested this amp first using a separate power source and current meter, and at normal levels it seemed to bounce between about a half amp and no more than two amps. I do not have any idea how much current it's okay to take from the game power supply in a Stern or otherwise, but this amount appears to work fine. I do believe in this configuration you are unloading the internal game amplifier, so some of this use is offset by less current use in the game's internal amplifier. Note that the PSW-10 solution, either internal -or- external will not have this potential pitfall because it connects to 110VAC, not the internal 12VDC.
--Donnie