Have you tried the other Roland piano patches other than the default? They do have brighter piano patches. Everybody test keyboards differently. Because piano sounds are so important, I usually test them for at least 10 minutes per piano patch. If you spend less than 20-30 minutes on a keyboard, you haven't really tested the keyboard. Of course, if you play the patch at ppp turning the volume to max, you get a loud muffled sound. That's how it should sound. You should set the volume to middle, then play from ppp to fff. How I test a keyboard:
1. Select default piano patch - first of all.
2. Play generally - pop and jazzy. Idea is to get a quick feel of the sound and touch. Play slow and fast improvisations to get a feel of the key response.
3. Then play a very soft piece (e.g. Claire de Lune). Followed by a very loud piece (e.g. Grieg's Piano Concerto opening - good piece since it ranges all the way from the highest notes to the lowest notes; or my favorite: Warsaw Concerto).
4. Play very low notes (lowest octaves) from ppp to fff (usually octave tremolo). Then the highest notes from ppp to fff.
5. Play individual notes esp low and high ones and let them decay naturally. Play them ppp to fff as well.
6. Play around with sustain pedal and soft pedal and listen for sustain resonance (where applicable).
7. Play around quickly again on any jazzy or pop piece to get a general feel again (usually my own composition

since I usually write with frequent key modulations that let me cover keys involving lots of black keys - to get a feel of the touch and response again).
And that's just one patch. If I'm really interested in the keyboard, I would do that for all the piano patches (otherwise I would focus on the main sounds). Then focus on the "best" patch and play with it more. For other non-piano patches, there are different ways of testing, but too much to put down here (since we are focusing on the piano sound). But I would still concentrate on the bread-and-butter keyboard sounds - E pianos (focusing on the DX sound that I love, and the Rhodes), organs (esp B3 with focus on the leslie effect), clavs, and strings. If it's a stage piano, I would spend quite a considerable time playing with layering piano/e-piano with strings/pads, trying out different combinations and playing around with the volume control for individual layer (testing mainly for how good it mixes, whether the fader is long enough for me to control the dynamics comfortably, and indirectly the polyphony - making sure there's no sudden polyphony cut offs with normal playing). For polyphony,
never trust the specs. It depends on how the patches are programmed. A patch heavy with combined elements runs out of polyphony quickly when layered. So always test polyphony in real layering playing conditions.
CP300 is not light. It's actually quite huge because of the speakers. You can get a USB cable from any computer shop. About software, it's a new topic entirely. Perhaps in another thread? But you should just focus on the keyboard first and expand into software later. You'll need to consider your computer and specs, audio interface, the actual software etc - which means more $.
I have a S90. Old already. But I got it because it's non-graded. That's important because I do sequencing of other things other than piano. I needed one with even key weights.
As what I wrote in a thread long ago, the problem I have with the CP300 is the volume faders. The resting position of the faders is in the middle, not the lowest "zero" position. There's a "click" in the fader when you move past that middle point. That's very silly of Yamaha to do that - the only keyboard that does it!!!! They don't do that for their other keyboards. It makes controlling volume of layered patches uneven. And I use layering very often - faders and mod wheels are very important for me. But other than that, CP300 is a beautiful instrument. Esp the speakers...can't praise the speakers enough! The reverberation...ooohhhh! For home, it's almost the "perfect" keyboard. But for stage, built-in speakers are no longer important. Here's where Roland is going to shine because of the ergonomics. Roland stage pianos are built for the live player. They have thought of everything to make the live player control and play the keyboard well.