Your objective is technically sound and worth pursuing. What you are
considering is the addition of a modern plate amp. Adjustable gain,
variable (active) x-over, standby, phase reversal and thermal shut
down. Normally, active subs have a factory-cut access, isolated from
the main box, in the back to allow for a clean, (airtight) recessed
installation. Adding this box in a box will displace some Vb possibly
effecting port tuning of vented enclosures.
If you could tolerate exposing the depth of the unit, allowing it to
protrude off the back, maybe 3" or so, you could avoid cutting
an access mounting box into the sub. You would need to build a
simple pine frame with (2"x2"'s or 1"x2"'s). As long as you clear the
circuitry, behind and around, while allowing sufficient surface area to
accept the screws to hold the plate amp, you're good to go. The only
trauma to the box would be the screws holding the frame to the
cabinet. Paint the frame black. Just remember its' there when moving
the sub around. Otherwise you would never see it.
The place to do this is right over the existing speaker posts. Most
passive subs are designed to sum the left and right power output from
a stereo amp. The fixed x-over circuit rejects the highs (to the subs)
and passes the lows to the driver. A driver can have a single or dual
voice coils. This would be determined by observing the wiring leaving
the x-over after removing the post plate. You need to by-pass the
original (passive/fixed) x-over network. I would utilize the original
post system to transfer power from the plate amp to the driver. Leave
the wires attached to the speakers but disconnect them from the x-
over's output. Observing polarity (red-black) join those wires to the
wires leaving the the post(s). If you have a single voice coil, only one
post set (left or right) would be needed-- electric tape over the other
set to avoid confusion later. Dual voice coils, use both post sets, just
branch out in parallel from the plate amp output Now you have some
place to plug the output of the plate amp once mounted to the
frame. You will need to consider the voice coil arrangement
(impedance) to determine the proper power rating for the plate amp.
Only a more experienced DIYer would consider cutting and installing
an access into the cabinet. It's done all the time, but many more
aspects need to be considered, including removing the driver, the
stuffing, displacement issues and (re)sealing integrity.
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