I wouldn't touch a RBX personally, had 2 mates who owned them and both started sounding really crap, kinda farty sounding, after about 18 months or so. They sounded awesome before they fucked up though.
just got me a yamaha trb 1005 fretless, played the fretted version a few times.
Really cool instrument, very versatile and you can get some heavy middy tones.