I have a spare box that I can make a front baffle for
just to see what the sub output of the little Yamaha
would do-- for grins, as we say. In my general ignorance
of things, I wanted to try the isobarik technique to get
that constant pressure volume mentioned in Ray Alden's
book Speakerbuilding 201.

But yes, I want to do the Tuba 18 and am enthused about
it.

I have never used a sub system before, so this is
just daydreaming.

The Roland 8 inchers have pretty small magnets too.

I'm wondering what one of those cheap plate amp subs
woudl so in such a rig?

--> The thing I want to avoid at all costs is any sort
of rumbling, wall shaking thud that may be a cause for
gee-wiz, but sound like shite on music-- in other words,
what most people experience as home theatre. :-/

I'm pretty happy with my single driver MLTL boxes using
Fostex 127e from a design at Single Driver Forum. It is the
best scratch build I've done, but still doing some tuning
stuff on those.