I'm not sure why you weren't able to record the synth (the Blofeld?) using the AG06, but it may have been from using an incorrect cable. The Waldorf and Kawai both have stereo output, although I'm not certain whether the L and R channels are separate or combined on each. A guitar and a microphone both have a mono signal, so a TS cable and TS plug that lets you successfully record a guitar or a microphone isn't going to work for recording the stereo output of a synth.
IIRC from when I looked at the Kawai last night, I think it has unbalanced TS outputs, so its L and R channels are separate. That means you can either use a dual-TS-to-TRS cable to combine its L and R channels into a stereo signal, then plug the TRS end into a stereo jack on your interface if it has one; or you can use two separate TS cables and plug each one into a mono (TS) jack on your interface, such that you're recording the Left and Right channels to two separate mono tracks in your DAW.
The specific types of cables you'll need will depend on the types of output jacks on each instrument, as well as the types of input jacks on whichever interface you choose.
For instance, I believe the PreSonus AudioBox iTwo has two mono inputs but no stereo inputs, so that means you'd need to record the L and R channels of the Blofeld or Kawai as separate mono channels. It also means you'd be able to record only one instrument or the other, but not both at once, and you also wouldn't be able to record a microphone at the same time as the Blofeld or Kawai (but you could record a microphone at the same time as a guitar). So if you wanted to record, say, your guitar, your voice, your Blofeld, and your Kawai in the same song, you'd need to record everything in at least three sessions: (1) voice plus guitar, (2) Blofeld, and (3) Kawai. Of course, it doesn't matter whether you record them together or separately as far as the end result is concerned, since each one will be recorded to one or two tracks in your DAW anyway. But if you think you'd like to be able to record your voice while playing either the Blofeld or Kawai, or record yourself playing the Blofeld and Kawai at the same time, then you'll need an interface that has more inputs than the iTwo has.
When you're shopping for an audio interface, here are a few questions for you to think about:
- Will you ever want to record the MIDI output of your Blofeld or Kawai instead of their audio output? If so, you'll want an audio interface that includes at least one MIDI IN port, and possibly two MIDI IN ports.
- How many mono and stereo audio inputs will you need? Two mono inputs are enough to record either two mono sources or one stereo source, but you might want to get something that includes one or more stereo inputs.
Obviously, the more inputs are included, the higher the price will be, so you'll also want to have a clear idea of your budget-- the amount you'd prefer to stay under if possible, as well as the maximum amount you're able to go up to if necessary. That price range will help you rule out some of the possible contenders and narrow the field quite a bit.
