I wouldn't go any smaller than 1.5 cubic feet. Going smaller demands more from the amp as the servo system tries to maintain a linear output, and the smaller box fights harder against it to doing that.
That is why with less power then box can be bigger and with more power the box can be smaller.
The A300 does not have the rumble filter or the additional 12db per octave selectable slopes at 50Hz and 80Hz.
Sorry to rehash this old thread - but I would like you to elaborate a bit more if you could.
If you take the T/S parms for the 12-04 and put into a subwoofer design software (assuming this was a conventional subwoofer - not a serveo design)- you get only 25 litre needed for a Q of 0.7. With 1.5 cu ft (42 litre) its more like a Q of 0.56. I would think the servo would work for both of these designs with the only downside - less output for the smaller box (and less efficient).
I'm interested in the most (musically clean) output for the smallest footprint (WAF). So while the 12" is already a pretty small box - I am wondering the same questions as the original poster - If you reduce the size of the box - what are you giving up besides max output? Thanks as always.