You can use an 8 ohm or 2 - 16 ohm speakers in parallel or 2 - 4 ohm speakers in series as long as you keep the impedance at 8 ohms. The included fat boy speakers are a joke. I have bought some very nice speakers from ESU, but any speaker you can find that will fit in your loco will work. Remember, the bigger the speaker the more air it can move and the louder the sound. Also bigger speakers can produce better low frequency sound.
Enclosuers are also important, you have to isolate the sound between the front and the back of the speaker. The front of the speaker produces a positive sound and the rear of the speaker produces the opposing negative sound wave. If you do not isolate them the 2 waves cancel each other out. I’ve been building my speaker enclosures out of 1/8 inch Lexan, I’ve gotten very good results.
