I've just tried a solution that I can confirm works with Filtatron -
at least with an iPad 2 (but does NOT work on iPhone 3GS, which I currently have, but IS supposed to work with iPhone 4)
The TASCAM iU2 audio / MIDI interface has a built-in (rather short!) dock connector to hook up with pre-iPhone 5 devices.
(Have no idea if the adapters for the new iO device connectors will work or not; I probably won't upgrade the 3GS until after the first of the year, and my iPad 2 should last quite a while for me...)
It can also connect thru a powered USB hub using the Apple Camera Kit (which unfortunately also does NOT work on the iPhone 3GS
).
If using the dock connector, the interface can be connected to a USB charger (connected AFTER the dock connector is attached!) to charge the attached iOS device.
MIDI is standard DIN connectors using included adapter cables.
Audio IN is 1/4" instrument, and L/R (stereo) 1/4" line / mic (plus includes 1/4" to XLR adapters for microphones)
Output is 1/8" headphone, stereo RCA, and RCA digital S/PDIF, with level adjustment knobs for just about everything.
Sound quality is GREAT - all my audio apps on my iPad 2 will default to the iU2 when it's connected, and all work fine.
(Animoog sounds AWESOME going out the S/PDIF to my computer's audio system!)
Audio in to apps is very clear, and signal levels are NOT an issue~
As a test, I ran the same Filtatron sample on my 3GS and iPad, feeding the audio in on the iU2 from the 3GS headphone jack.
From Filtatron on the iPad, I could tell very little difference in audio quality when switching between the "sample" Filtatron audio and the app line-in of the same audio on the 3GS (but it did make for some crazy effects with the mix of two Filtatrons running simultaneously!!!).
Now the bad news:
Even though this device has been out less than a year, TASCAM (and Sweetwater.com in the US) have already dropped it, and from the traffic on KVR and other forums, it appears this COULD be because of potential issues with the post-iPhone 5 devices and the new connector.
(I have a feeling that will be the issue for a lot of "legacy" 3rd party hardware already in circulation, unfortunately
).
However, Guitar Center still carries them, which is where I checked out and wound up buying mine.
(Would't be surprised if they're available at a big discount before too long, but I don't know that for sure.)
No regrets on my part -
it's a great portable I/O solution for the now "legacy" iO devices, and since I'm generally NOT one that feels the need to "upgrade" every year, I think I'll get plenty of use out of mine.
Just developed a whole new appreciation for things to do with Filtatron!