I used to play in a VERY heavy death metal band with the same bass. I had no problem with the sound, infact i thought it was great. I then started using the Ibanez SRX300 which is very similar to the rbx, just a thinner neck and a higher output. I changed because i didnt like the feel of the yamaha, it simply didnt play fast enough. I think Ibanez would be the best way to go next, SRX or SR both are great fo that type of thing.
Im back to yamaha now though, an older BB and TRB. I play in a stoner/doom band using drop c, and i use fairly heavy strings (55-110) and they make a huge difference. Just try a few things out.