I recently bought the GPA 604-III with GPA standard cross-over. Very pleased with the result.

Used a large (200 litre, 7 cubic foot), ported cabinet based on Stonehenge V, but modded per Todd White's suggestions. Lovely Palesander veneer over double-walled 19mm ply. Rubber carpet underlay on three inside walls.

For something smaller, go with the original Altec 620 cabinet, here.

Andrew