First off: Thanks for everybody's feedback, it's much appreciated !!
@Capoeira: Thanks for the suggestions, I've tried Bristol before but it was VERY slow to react to my keypresses. I tried tweaking the settings but couldn't figure it out
I haven't heard of Amsynth but I'll try it. However: NI Massive is not a "simple" synth as you say. Or - if it is - it is VERY well tuned. I've tried hundreds of Windows VSTi but none come close to Massive for gnarly, dirty and rotten bass sounds. Hence why I bought it.
@Slowpick: Reaper is nice but I was really hoping for a non Wine-based solution. If all else fails I will reevaluate it, though. I will definitely give it a shot just to see if Massive would work.
I've also noted Alphatron and the Bass-Rack (we don't have Computer Music Mag here, though). Phasex sounds interesting, too, even if it looks a bit too "scientific" at first glance. I usually try to avoid those as I end up twiddling knobs for hours and getting nowhere

Yes, I do understand synth basics, but can't be arsed most of the time, lol.
I already had a look at Qsynth in the meantime and it looks like it might be what I need. Too bad that many audio softs really look like 1999 in the Unix/Linux world, though ... The Calf Monosynth is also noted but what are those rakarrack FX ?!
@falkTX: First off: Many thanks for cadence - it's awesome !!! Is there any hope that dssi-vst will get fixed for Oneiric ? Also thanks for your suggestion regarding VSThost setup - I'll try it. As for TAL-NoiseMaker: I've been seeing that in some videos but haven't tried it. Thanks for the suggestion and the reminder. However, regarding Bristol: Like I mentioned above - the Bristol synths are extremely slow to respond on my machine (which makes then unusable). Do you have any idea what I might be doing wrong ?
Also - have you considered offering Ardour3 Beta with KXstudio or did you decide against it for particular reasons ?
I still haven't tried AV Linux, I'd really prefer to stay on Ubuntu as I also want to use the machine for other purposes (and I like the candece tools) but I'll give it a shot from the CD at the weekend when the wife's not in the house and I have time to mess about

Is anyone actually working on a "receptor"-like clone ?