ER1
EA1
ES1
EM1
The ER-1 is a drum synthesizer. What this means is you are given wave and envelope shapes and you select the depth and such to actually create the drum sounds, which you can then sequence using the pattern sequencer. Luckily there are few parameters so it is quite easy and fun to program, however you'll only get really electronic sounding beats, like Kraftwerk. In fact, very much like kraftwerk and minimal techno style stuff. Not to say it can't be used for other things, but don't expect anything that sounds anything like a real drum to come out of it.
I have one of these here. It also has a delay which can be used only across all of the parts, however you can tune it to get some funky bassline-like effects out of it.
The EA-1 is a bass synthesizer and sequencer. I believe it has two parts, each of which is sequenced separately, so you could have a bass and a lead, or something else. It is far from analog-sounding, but uses analog synthesis similar to the ER-1, just with more features and less parts.
The ES-1 is a sampling sequencer. It isn't based on analog synthesis, but it can record anything you input (including the kitchen sink), and you can use that as a sound. It has a variety of sounds already on it, not just including drums but also guitar strums, synth bass, acoustic bass, vocal samples, etc. You could craft an entire song with this if you had a bit of patience, but it is also very immediate and great fun to play. Despite not being related to analog-style synthesis at all, there are still plenty of knobs to tweak to change the sound.
I have one of these here (the mkII). It too has a global delay effect but also an assignable effect processor which can do reverb, delay, compression, distortion, phasing, flanging, bit-crushing, etc. Very fun to play with.
The EM-1 is kind of a cross between the ER1 and the EA1. It is the precursor to the EMX. It has two synth parts on which you can program monophonic bass and lead lines, or anything else. It also has a number of drum parts. I believe there may be some built in ROM samples too so that you can get some more dynamic and realistic drum sounds out of it, but it isn't a sampler. If you have this you really don't need the EA1, and probably not the ER1. The ES1 is still a good match though for sampling interesting sounds.
In all of these you can program patterns, and then string them together as a song. The sounds are actually a part of the pattern - they are not part of the song, and they are not separate parts that you reference. This is great because it means you can create an entire different sound for each song. And even change sounds between sections without any noticeable lag or cut.
They also play great live as once you have hit play, you can scroll through the patterns but it won't change until after it reaches the end of the bar.
You can sync them up via MIDI in such a way that they'll change to the same pattern at the same time, but setting it to ignore everything else (except clock) which is very useful too for creating an entire song workstation out of a bunch of these units.
You could run your Kaossillator through them. For example, I noticed one of the interesting features of the ER1 is that you can put audio through it and turn it on and off for each step, so you can get interesting gating patterns. Still, if you have more than one of these in combination with the Kaossillator, you'd be better off with a small mixer to connect them all to.
You could connect through a guitar amp but they don't reproduce sound in any good quality. They are designed only to amplify the range of sound produced by a guitar, and also to make the guitar sound 'better' - there isn't really such thing as a 'clean' guitar or bass amp.
Keyboard amps are again only really useful if you're practising with a band - otherwise you're probably listening in mono, to something on the floor, with a more limited frequency range. It's best in stereo with a good frequency range for any kind of actual music production.
For the best quality you should be looking at studio monitors, or at least a decent set of PC speakers (the type that come with an amp/sub unit are great, as you'll get some awesome bass). That is what I used to use before I got more serious and invested in some studio monitors. You would probably need a pair of 1/4in jack>RCA adaptors though, or a cable that does the same job.
You can connect headphones too, but you'll need a 3.5mm to 1/4in stereo adaptor if you don't already have one.
-- a Kaossilator won't.