So my other question is what would be a good upgrade for the
FE127e that would be plug and play for my MLTL?

FX200 has been mentioned by GM as a drop in replacement.

Overall I'm sold on the MLTL config. The boxes have nothing
inside and I extended the port tube to 6". Original calls
for much less to attain very low frequency but the elongated
port brings the harmonics up to where all stringed instruments
like double bass and piano sound superb.

I will be interested to see what you come up with for these.

I originally got the FE127e's for computer speakers. But they
sounded better than the off brand mains I had so I started to
build with them.

The previous bookshelve speaks had junk in them near as I
can figure but the workmanship is great. I have some
blank baffles made to tinker with some other speaks for those.
Measures are very close to what Jim Griffin uses for his
Jordan BR.