I'm wondering just how loud the environmental noise (wind plus vehicle) plus the radio sound is. My guess is that the sound levels are close to, if not in excess, of levels that are likely to cause permanent hearing loss.
There is lots of permanent hearing loss these days just from wind plus vehicle noise. For example every motorcyclist (with helmet) who travels at more than very modest speed (40 mph) is likely to be losing hearing. Most of us always wear earplugs for this reason.
I understand this. I'm a stickler for hearing protection. I use the foam plugs or highly rated muffs routinely -- like just for vacuuming with the shop vac or using the blow gun on the air compressor. It was actually riding motorbikes that got me into ear plugs. I never went without them.
In the vehicle, the noise levels aren't that high. If they were I would be using a headset. What I wrote was that it can get loud offroad, but it's not always loud. Most of the time I can talk to passengers in a normal voice, but bumps and so on can momentarily create disruptive noise. Does that make sense? There's not much constant-level noise (the car is fully enclosed and heavily sound-deadened with dynamat, and has two mufflers in series). But momentary noise can disrupt speech if it's not at a high enough volume.
My objective is not to overcome a ton of constant environmental noise, but to bring the radio volume up without excess distortion. The problem with 3 watts is not that the SPL is way inadaquate, but that by the time you get to 3W, THD is at 10% and the audio quality is very poor.
With the speaker under the seat, I do need a higher volume setting, but I'm not trying to crank this thing super loud
Anyway, I'm just clarifying. This thread is getting old and I haven't done anything because I'm procrastinating spending the money. I did appreciate the replies that helped me sort out my different options. I think I'll try a KES-5 without an amp first and see how it goes. If it's loud enough with only 1W from the built-in amp, then that will be fine for me. If I'm still having to crank it to 3W, then I'll probably need a small audio amp.