What is Tight Bass?

Tight bass is very simply: when the signal stops, so does the woofer (in fact, the entire loudspeaker). To not "stop on a dime" is to produce "overhang". In the world of loudspeaker design, there is the concept of "critically" damped woofers, as well as over-damped. A Google search will lead one to lots of literature on the subject.

IMHO "tight" bass refers to the system's ability to cleanly differentiate [in a mono mix] between a double bass plucking and an underdamped bass kick drum beating at the same time without mixing the two up to any degree. many systems can't do this as well as they ought. 

Tight bass relates to timing.

When a Bass note is exercised if it reverberates or cant musically keep up with the song its timing is off and it's called not tight...

You could add "tunefull" and you bring in timbre and definition as well.

NAIM = Tight Bass!

But be careful folks.  I tightened the bass so much the treble popped out and broke on the floor!

Well I thought that was funny but it's pretty early on a Saturday morning. 

Anyway, I have 3-series Maggies and those actually pump out beautifully tight bass, but I have found that to get the SPLs I prefer (music always sounded best to me in clubs), I use a swarm of 4 subs - 2 in the 10 inch range and 2 smaller ones.  Primarily RELs that are geared towards music. They are each dialed in such that you don't perceive sound coming from them, but together they subtly control the room SPL while reinforcing the bass from the Maggies. I find this method more all enveloping than the brute force of a single big sub.  Give it a shot!