I build the midibox16, and I have to say that this site/box/concept/thorsten is in my top 5 important things on internet !
I saw the midibox64 seq prodject, but 64 pots is a little too much for my pocket 
BUT I realy neet a simple 4 track midi sequencer for controling my rack-synth and not bring the PC to some gig I might have ;D
So, can I build some of the MB64 seq and then somehow, to load a midi file from the pc to the PIC ?
I would wait for the MIOS release of the MBSeq. There’s much coming (as far as I know the Seq will also be able to read Dump files from the PC). I’m waiting, too, at the moment… ;)
Besides: I plan to make the MBSeq 3 HE rack design ( 3 HE !!! ), which is only like apr. 5 cm deep (the cables connected to it will NOT stand up and so make it higher than it was meant to). It will be a little more compact that way though you shouldn’t do that if you have big fingers… ;)
When I’m done, I would put that file on the portal and tell you, if you’re interested.
so, should I start building the MB64 core ?
Yes, you can.
You also can build following modules, as they will be certainly used:
2x AINx4, DINx4, DOUTx4
Greets
/edit: Naturally build the core with an 10MHz Oscilator! For that PIC 18F…