Okay kids, a couple of things going on here.
First of all, about the teeny-tiny amps: it's really hard to make any solid evaluation of your tone when you're playing through underpowered equipment. So... your evalutation of your sound is subject to the frequency response of your little practice amp. And trust me, it isn't really good.
On the flats vs. rounds thing: after 35 years of playing bass and having owned every kind of bass and strung them with every kind of string, I can share this. People will say that flats have a "stronger fundamental," but this is utter nonsense. I would love to see a scientific comparison with objective equipment of the the frequency responses of flats versus rounds. I would bet a paycheck that the fundamental content is EXACTLY the same. The difference is that the rounds will give you a broader spectrum of harmonic overtones ON TOP OF the fundamental... which tricks your ear into thinking that the flats have more fundamental content. What they really have is LESS of the other stuff.
Real world consequences: flats work for some situations and some styles. If you want a big, thumpy, whomping tone with little upper harmonic activity, flats are the way to go. If you want to hear the higher portions of the harmonic spectrum of your instruments, you go with rounds.
Don't let anyone tell you that there are any sort of requirements about which you choose based on the style of music you play. I play active basses with rounds in a country/Americana setting, and they sound freakin great. I've heard flats played in a metal setting that sounded great. There are so many other variables in your overall tone that you need to experiment with different strings to find the tone that's perfect for you.
Yes, this is an expensive quest.
But for starters, you should be doing your comparisons playing through an amp that can actually reproduce all of the frequencies that your bass pumps out. A bedroom practice amp is not going to give you a true representation of your instrument's tone.