its doable but you would have to learn a bit more about the hardware on your own. the combination you describe is very similar to what we used to prototype.
there are different devkits. if you mean this one
then that is pretty close, it has the same amount of SDRAM, but you will have to write your own code for audio I/O using AD1871 (ADC) and AD1854 (DAC) instead of AD1939 (codec.) SPI init code and slave boot should be pretty easy/similar. not sure about SDRAM and clock init code, but there are plenty of bf537 sample applications if you search for them. most are written for the VisualDSP compiler suite but blackfin.uclinux.org has tips for porting to the bfin-gcc toolchain. (in fact IIRC there is generally more sample code for bf537 than for bf533.)
hm, you would also be missing half the audio I/O channels and all the CV output. getting the audio daughterboard would solve that.
the avr32 side might actually be more difficult as there is more hardware-specific stuff for all the peripherals. you would have to build out the screen, encoders, CV input, serial I/O and probably other stuff - for example, i can’t remember if any of atmel’s devkits have that much SRAM, or if any have both a USB device and a host port.
application code (e.g. bees, prgrm) is actually quite portable, just doesn’t do much without the low-level stuff provided by aleph avr32_lib (timers, CV in, screen, usb drivers, filesystem, SPI drivers, etc)
so i dunno, maybe that sounds like a good time to you, maybe not! or maybe just focus on the DSP part and follow the (very simple) param change / boot sequence serial protocol, for aleph compatiblity. (this is another way of phrasing trent’s suggestion. i too like the STMF32 discovery board, although i’ve run into a surprising number of bugs in the the STM peripheral libraries. if you’re developing your own control application you could use any microcontroller powerful enough for your needs.)
EDIT oh, just clicked the link. that looks nice! cheap and minimal with easy access to pins. but the same caveat applies as far has having to source quite a few other parts to get a working controller section together. and you would definitely need to add SRAM and configure the driver for it, which i found to be relatively tricky. bonus of that board vs. an atmel devkit is that it looks relatively easy to have a totally arbitrary pin configuration (thus, you could closely match the aleph board), the downside is you have to add pretty much everything yourself. which would be expensive and time consuming but certainly educational.