I'm now looking at the CA2900. Would I be able to connect two mics to this. If so, what cables would I need?!
CA2900 preamp is intended for miniature "2-wire low voltage microphones", providing about 9V to them. Pairs of microphones which terminate in a stereo mini-plug work with it, the type of mics which are often run directly into the mini-plug input of small recorders providing Plug-In-Power. CA2900 will provide better powering than PIP of a recorder and possibly cleaner and more easily adjustable gain.
It does not provide "phantom power" (typically 48V) required for condenser microphones that use an XLR connection such as the AKG C 214, but your recorder may provide that directly, eliminating the need for the CA2900 with that type of microphones, or dynamic mics which require no powering.
Trying to get a really minimal, plug 'n' play rig going, to record my band as well as doing some 'live' home recording (à la basement tapes).
Although Mr Stranks' sound advice is de rigueur for live music audience recording, the above request sounds less to me like taping from the audience and more like on-stage or "home in the basement style recording".
Although the OP may initially be recording just two channels of ambient sound of his band performing out around town or doing their thing in the basement, by finding a suitable location to make an ambient location recording using the internal mics of his H4, it sounds to me like he is likely to be moving towards a more home-recording-like approach of microphones placed relatively close to each instrument and each instrument more or less given it's own channel, with the leakage between these channels providing sufficient room ambiance to glue the elements together and everything recorded to a single stand alone-recorder to keep the recording setup simple. And it will need to remain simple if he's also playing in the band or recording himself playing alone.
If that's the case, then a DR-680 is a good choice for recording 6 separate channels (8 if including the digital input). A small mixer will allow for additional inputs mixed to those recorded channels, but in that case one needs to "commit" to whatever mix decisions are made prior to recording. A recorder with more channels allows the flexibility to keep the recording part simple, at the expense of some mix-time afterwards.
Both small and large diameter condenser microphones can work for either style of recording. If a general trend can be called out, SDC (small diameter condensers) are generally more appropriate for ambient location recording (audience recording of a band performing in a good sounding space) because their directional qualities are more consistent for ambient sound arriving at the microphone position from all directions, and also partly because they are smaller and lighter and thus easier to use in audience taper situations. For independent mic'ing, positioned relatively close to the sources on-stage or in a less than great sounding room, dynamic mics and large diaphragm microphones find good application. In that situation, the microphones needn't be as much of an "well-behaved, unbiased, neutral observer" as is appropriate for audience recordings, and more specific microphone attributes may be a better fit to certain instruments. The sensitivity and detail required of microphones for good ambient capture of a "you are there" recording in a good sounding room become less important, and the resulting recording techniques instead are intended to minimize the influence of the room, instead creating a synthetic ambience from individual close-mic'd sources mixed together.
Plus, at a gig, the OP may be able to put up a couple of wide spaced room mics on stage for ambience capture, and simply have that routed back to the board through the house snake, so as to plug the 680 into the board and record that pair along with four other channels of the house's close-mics. Many audience tapers would like to be able to do that to make superior recordings, but don't have board access, yet that shouldn't be a problem if he's recording his own band performing.