soma advice do what you want with it.
Ditch the 505 it's basiclly crap. they're not exactly known for their flexebillity and sound quallity. In a studio situation when compared to a decent synth they tend to fall out rather weak. As you said yourself, deep intens basses.. forget it, only a weak emulation of..(I will get flamed for this I know, I don't care. Expect
the JP8000/8080 Roland didn't made any good things in the last few years, they're just abusing their name)
If your in electronic music you'll need to get a sequencer sooner or later. since version 2 cubase does midi very good. All versions up from there only offer more audio-related options. Note that audio in the PC will use a fair amount of system resources from your PC. Cubase itself doesn't offer you any sounds. it's just a sequencer. it do offer you extensive audio possebillities such as non-destructive editing, real-time FX and since version 4 also VST instruments, wich are emulated synths. Note that a good quallity VSTi (instrument) actually do cost some money and really do eat up CPU resources quickly, but quallity of those synths is of the same as any hardware synth. If you really want to use your puter for sound-creation and FX and audio recording, see you get a fairly fast one with enough ram and a big and fast HD. If you ain't got access to that a version of cubasis or a lower version of cubase will be equally intresting. Updating can amlways happen at a later time.
So some alternatives..
Yamaha RMX1 tend to get fairly good reviews and is in a similar concept of the 505. it do sound better and it has been putted together in a rather intelligent way.
MPC2000. A sampler + sequencer. You could use this as a standalone machine. It has no onboard sounds, but you could sample whatever comes in your way. Almost ALL d&b basses are samples and filtering is not seldom done by the sampler too. and in fact 90% of the drum and bass rythms are done with a sampler.
The best option IMO: A full blown rack sampler with cubase on the puter. Something like
the akai s2000/3000 or
the Esi2000 or
the yamaha A3000/4000..(or one of the top models of akai/emu/yamaha). well something with balls. For drum and bass a sampler is from vital importance. you also could consider a software sampler such as the ESX, but you'll need a fairly potent puter to get away with it. Alternativly there are PCI cards available with onboard DSP I think creamware pulsar and stuff can be of use too, but I'm not really at home in that kind of semi-hardware stuff.
conclusion.. get a sampler and an old version of cubase and you're off. If you have a little more money get an update of cubase, a copy of Recycle, a mixer, and a keyboard and some FX boxes etc....