Dave,
Just got back from four days of sailing, the weather sucked, and still does. Sailing home in torrential rain and thunder-showers is a bit intimidating.
The kids/adult-children are beautiful, and I know you are a proud papa.
The bad news is that you got old, and like most of us that have been in this business for more than a couple weeks, your hearing is probably shot to hell. It comes from all those "In your face" sounds at ear-bleed levels. Consequently, we, the males of the species tend to loose our high frequency sounds, while in contrast the females loose their low frequency hearing abilities. It would be a good idea to have it checked out by a reputable audiologist.
In reality, even if you doubled, or even tripled the internal speaker's amp power it probably would not make a discernible difference it what you hear at the player level. I do, however, know individuals that have changed out the speakers on the PSR-S900 and achieved incredible results. This, of course, is quite expensive if you cannot do the job without the aid of an electronics repair shop.
As you know, I rely heavily upon my singing--it's essentially my right hand voice during most of my performances. I stopped using the keyboard's internal speakers when I first purchased the Bose L1. I did this for two reasons. First, and foremost, that was back when I was still using a hand-held mic, and there was a problem with feedback from the keyboard's speakers that affected the entire sound system. When I dead-plugged the keyboard speakers the feedback problem vanished, my vocals sounded much better, both to me and my audiences, and for the first time I was hearing exactly what my audiences heard.
Finally, the keyboard's line-in, at least on the PSR-3000 and Tyros-2 is more than adequate for the output of the TC Helicon Harmony-M, which I assume is similar to your system. The only complaints I've heard along this line was from folks that have failed to set the tiny switch on the back of the keyboard to Line-In. That said, the Auxiliary Input, which is a separate function, at least on the PSR-3000, is just plain awful and requires levels much higher than Line in order to function. Some players have actually placed a small pre-amp in-line with the output of devices that would be fired into the auxiliary input just to achieve the needed operating levels.
One day in the not too distant future, I'm hoping we can get together again, drink a few beers, and maybe compare systems and setups. You have always helped me in the past, so maybe I can return the favor this time around.
Good luck old friend,
Gary