Yeah, I started off today by going to radio shack and buying the pot and knobs to do that hack. But after I got my marquee off and was looking for the best place to mount the sound-pot, I started thinking about the old amp sitting in my basement (it came out of a car I sold ~10 years ago and hasn't been used since)
So I decided I might give it a shot. I got it powered up and I hooked up the mono speaker in my cab and it sounds GREAT! In fact, I really like it better than my 3-piece logitech setup. Sure, the logitechs probably sound better with music, but the mono built-in speaker really makes it sound like I'm in an arcade. Part of that is physical positioning and where the sound is coming from, but part of it is "quality", and I have to say I'm really digging it!
Clearly the decision is up to you, but I have to give my vote for the authenticity of the speaker in my cab.
My amp has a 'gain' setting which I have all the way down as low as it can go. I also (currently in my rigged-setup on the floor) only have the left channel running to it. But MAN, it's already plenty loud. The car amp really is overkill. Maybe I can probe how many amps it's pulling at its lowest setting....