Depending on the equipment- bubble wrap alone, can be too fragile, while foam sheets can be too rigid and unyielding (it doesn't dissipate shock loading well).
I've done packing, where I put a layer of foam board around the equipment (like "armor"), and foam board lining the box. Between the two- I put sheets of bubble wrap, as a more compliant cushion.
Since the foam board increases the surface area of the equipment, and also provides a smooth surface- there's no "hard points" to pop the bubbles in the bubble wrap.
I sent a Sansui 9090 receiver by USPS ground, from Atlanta GA to HAWAII that way once. Made it without a scratch. In that case- I punched holes in an inner piece of foam board, so that each knob and switch tang had a cavity where it didn't contact any part of the packing material- the foam board only contacted the faceplate flat surface itself. I then put a second solid piece of foam board on top of the one with the holes, to prevent intrusion by the bubble-wrap to the knobs and switches. The padding material wound up a total of about 3" thick all the way around.
Yes, it was expensive- but it worked flawlessly.
Speakers are generally pretty smooth flat surfaces, and not nearly as dense as electronics. Cover the grill with cardboard (to prevent intrusion), then bubble wrap layers directly on the cabinet, with a layer of foam board lining the box (around the bubble wrap) will usually work fine. Just be sure to use at least 2- 3 inches thick total packing material thickness (bubble wrap and foam board combined), so that there's something to absorb shock loads.
However- in this case- a personal car is a LOT more gentle than a shipping truck. I'd still want to "armor" the cabinets with a layer of cardboard, then wrap them with something like towels or bubble wrap, a few layers thick, and then put them in a box. This is just to prevent errant objects from intruding and damaging the drivers or finish.
Regards,
Gordon.
