2-way speaker with best bass

The original Large Advent has bass appeal I've never heard bettered. If units sold is a valid measure of most successful, these are da bomb.

The problem of bass is that it requires a large driver and a large box. The
problem with midrange is that you need a small driver to produce this
properly - that is without "beaming". You can get prodigious bass
by adding ports to a small woofer but it won't be musical - mostly one note
bass. To get decent bass you need critical damping. To get decent mids you
need proper dispersion (not beaming) and a dome tweeter would be good for
dispersion but it can't handle the excursion necessary to crossover low
enough to a big woofer.

This is all physics.

Given the constraints - there are several ways to go about it.

1) A large driver with a whizzer cone - the draw back is distortion and a
bumpy frequency response in the mids.
2) A small but very powerful woofer (6 inch) in a sealed box with a large
phase plug or dome to control dispersion better in the mids. (You still won't
get much bass but mids will be better and what bass you get will at least be
accurate)
3) A large driver for bass and with a single horn for mid and tweeter. This
allows you to
crossover low down so you can avoid beaming from a big woofer. You get
good bass and great mids with a gentle roll off in the highs. All round pretty
good except you now have your crossover in a rather critical range (700 to
900 Hz)

Thats pretty much all folks - only so many ways to skin a cat. You can play
with TL and bass extended ported designs until the cows come home but
IMHO you will never get the "best" bass response that way.
Although to many people "best" may simply mean "prodigious" - in which
case anything is possible.

The Merlin VSM mentioned by Pubul uses active eq to set what is IME the standard for extention/definition from a reasonably neutral, smallish 2 way box. There certainly could be a non eq'd 2 driver, 2 way out there that can match the VSMs, but I definitely haven't heard it.

OTOH, if you want parlor tricks -and can abide more deviation from neutrality - the original Red Rose mini monitor skillfully gooses the midbass to enhance its already impressive low end output (for a tiny box) and generate a better illusion of bass than anything I've heard remotely near it's size.

The Gallo Ref3 series uses 3 drivers in a 2 way configuration (the tweeters run full range) and allow use of a 2nd amp to "co-drive" the woofers on a second pair of voice coils. These may be the champs per your specifications.

Finally, as a technical matter, you could mate a subwoofer(s) to a single driver speaker and meet your description. To give some perspective, I get flat resonse +/_ 3db (relative to 80 db) from 25hz to crossover from a pair of Velodyne SPLR 8" subs run through an SMS -1 cross/analyzer/PEQ. I use a number of different mains, but I haven't yet tried a single driver. I'm also told that the Magnepan MMG wall (which can be stand mounted) and some electrostats also operate without a crossover. This set-up will win hands down!

Marty

Here's a vote for the Silverline SR 17.5.

Actually, I think some (or all) of the M-L hybrids meet my last description.

Marty