Reli,
BTL is definitely better, especially in boomboxes, and for more than just power. They typically have a higher S/N ratio and usually sound better when operated in the sweet spot. If you look at most datasheets, you'll find that distortion at lower operating levels are usually very good, under 1%. When you get to the 10% maximum, in my opinion, it might not blow up, but it's not really listenable unless you don't care about audio quality. So with BTL, you have far greater usable volume before distortion becomes an issue. For the TPR-950, don't forget that the fantastic sound and volume is accomplished with 6 batteries, yep only SIX, not 10. How?
Boomboxes have a huge problem, and it has to do with voltage. In home audio gear, there's plenty of voltage available. In fact, many if not most home audio power supplies have secondaries that are 55V, 70V, maybe even 90V. Boomboxes are limited by the number of cells and battery life. Also battery powered devices usually need to strike a balance between portability and weight. Sure, you can stuff 208 cells in it, so it can power a Prius for a few minutes and miles of travel but are you gonna have a crane with you at all times? The thing is, amplifier output is limited by available voltage. However, in a BTL design, it's possible to realize almost twice the power output compared to SE with the same available voltage. See the benefit to boomboxes and why it's better? How about those mini boomboxes that only have 3V or 4.5V, or 6V available?
So, you are probably wondering, well if voltage is such an issue, how do car amps can have such high power? It's because internally, they operate at much higher voltages due to a DC
C converter which boosts the voltage typically to 36-40v. Ok, I can already sense your next question... why don't boomboxes do the same thing? Because power is power. Boost voltage and you get increased current consumption. Means battery life is virtually non-existent, especially the DC
C converter is not going to be 100% efficient.
Now, about the STK-430-II which is what is in that Hitachi... it really feels right at home at very high voltage. Look at the datasheet for 13.2V for power output.... nothing special is it?
As for which boomboxes are BTL and which are not, here's a simple test if you have access to the speaker system, or the boombox has external speaker jacks:
Simply ohm the (-) speaker leads. On SE systems, a common ground is employed and you should read 0 Ω or close to it. On a BTL system, a floating ground would be employed so the negative speaker leads are not grounded together. While you might read some resistance, it won't be 0, or .9 or something small like that.