Yo PHosphene! The "cs" stands for "current sense", and represented a change in the electronics that I don't claim to understand. The oldest ones are designated VC1Q and twinQ, the later models VC1Qcs and twinQcs. All of these were made in England. Then business "issues" occurred. Studio Projects ended its relationship with the British manufacturing firm and began marketing redesigned units designed in the US and built in China. They are designated 3Q and twinQ. I owned a twinQcs, which was a very useful unit with a lot of character that never gave me any trouble, except for the add-on digital output card, which did not impress me.
I took a chance and traded it in on the new twinQ, and in general, I have not regretted it. The new twinQ doesn't do what the old one did, but it does a lot of other things better. I consider the old twinQ a one trick pony, mostly useful as a vox box. The new one has a little less character, if you will, but is a hell of alot more versatile. The EQ on the new unit is *much* better, and the iron switch allows it to be 2 very different preamps. This switch changes it from a transformerless to a transformer based solid state preamp at the touch of a switch. How cool is that? With iron off, it's a typical Burr-Brown type pre, not unlike a Grace 101. Very clean, no color, just the facts, Ma'am.
With iron on, it reminds me a little of my Avalon AD2022, another solid state pre with big mucking transformers. The compressor is very usable, but is the area where the pre is most different from the old twinQ. The older optical compressors have tons of character that you can't get rid of. When it's the right thing, it's perfect. When it's not, you're stuck with it. The new compressor simply doesn't produce that old agressive Joemeek sound, but for a broad variety of purposes, it's a better compressor. Overall, the new amp is cleaner and quieter.
How does it work on bass? No real clue. I only play bass a little, and when I record bass, I usually play it safe and plug it right into that Avalon, which is simply one of the best all-around bass DI's in the world. Mostly I use the Joemeek for vocals and percussion. That's just how my equipment options work out. And frankly, most of the time, I use it with the iron switch off. When I want that big-ass transformer sound, the Avalon is my best available option. Hope that helps.-Richie