I've been very pleased with my Bazooka 10" powered subwoofer.  It rides
in the back, but has to go when I go camping (unplugs and is easy to
remove) so we can use the bed.  I've also installed 6x9's in the lower
front door.  They fit fine with a partial cutout of the metal that's
down there.  They help, but you really need a subwoofer of some sort to
overcome the high noise level of the Vanagon.

An 8" powered Bazooka would probably do just fine too, and should fit
under the seat.

It will change your Vanagon experience completely!

--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT