Standard LF driver in L200 is LE15B (cloth surround), not LE15A (lansaloy surround), and the crossover frequency is 1200 Hz. I last calculated the internal volume as 4.9755 cuft. It' different every time I do it.

136A's will play nicely in that box with one of the ports closed for extended bass (~25 Hz), or both open for tighter, punchier bass (~35 Hz). I'm using 2235H's in them right now, with proprietary port closure technology so's I can choose bass tuning as desired.

You'll want to move the crossover frequency down to 800 Hz with, say, 3110 crossovers. The rear panel crossover cutout size is the same.

However, as stated above, if you're thinking L300, you'll not be fitting the longer horn in there unless you're willing to punch the LE85 out the back, a mess.

The long horn may be optimum, but JBL used the short one down to 500 Hz in the "legendary" S7 system.

L200B used the shorter horn crossed at 800 Hz with 136A woofers, and several members have successfully added 2405 UHF to them using 3106 crossovers. It can be cut into the baffle space above the present horn if you want to forego the external box.

There is no biamp input on L200 that I'm aware of, but you should be able to use your active crossover. Feed everything below 7 kHz to the 3110's, and the VHF to the 2405's.

The 8 bolts will remove the woofer, but it may require gentle persuasion to come free if it's not been removed before. Lay the cabinet on its back to work on it.

There's no "going into the back of the cabinet" option available.

See my avatar for an alternative formula for L200 upgrade if you want to get radical....