But the SK30 is pretty rare, so if you see one for a good price, I'd pick it up then. The SK is the big (with case the thing weighs 90lbs) brother of the Sk20. Basically it's a 20 with a mono-synth added. I am not aware of any sonic differences, but I don't have any of the others of the series to compare. I am aware there is a SK-50D, which apparently adds a second keyboard and this scares me. Here's a review of each section and some general specs below.

Mono-Synth: This section is comparable with another old Yamaha, the CS-5. Doing an A/B comparison, most non-resonant sounds were pretty similar. There are some major differences overall though. The SK30 lacks the smooth portamento that the CS has. I'd even go so far as to call it whiny and, for the most part, unusable. The resonance is also very different. The SK's has a more liquid quality and an increased effect on the overall sound. Lastly, the LFO is sine only, unlike the CS's 3 shapes. All of that in mind, for what it was intended for (bass and leads), it does very well. The overly simple modulation throws it out of the race for complex sounds and FX, but the bread and butter sounds are all there with a good punch in them.

Poly-Synth: Though it is a very solid sounding section, I would not use the word "beefy" to describe it, more like "spacey." It lacks any bite whatsoever and, even in normal attack mode, has way too slow an envelope to be used for any bass or percussion sounds. But, the pads and strings you can get certainly shine and glitter like Bowie a'la Ziggy Stardust (especially when layered with the String-Synth).

String-Synth: As a solo instrument, this is the worst of the sections. With very limited editing capabilities and a rather thin sound, it's only real purpose is to layer with the Poly-Synth. If you're looking for a string machine, you can do alot better than the SK.

Organ: Not being a fan of organ sounds, I was suprised at how much I liked this section. Its sound is all about the 70's era rock organ, especially with the Ensemble effect in use. The tone sliders give you quite a deal to work with in the way of synthesis. Within minutes of turning it on, I was able to get everything from mournful goth stuff to some tones which made me want to poof my hair and get some tightpants. The percussion is okay, I'd like to have a bit more "click" to it, but that's just me.