Amps that bring liveliness to low efficiency speakers?

It might help if you tell us what speakers and gear that you have.  The lack of liveliness could be due to a very warm sounding preamp/source and/or speakers rather than being inefficient.  But more watts on an inefficient speaker is always good.  If your have a very warm system, Bryston is probably just about the fastest responding amp I know of.

Pure Class A!
I read a review years ago on the Dynaudio Special 25’s..The most musical system was a Cayin Class A SS amp and the reviewer stated that because Class A is always "on" it allows saturation of the crossover and faster response time giving them much better low level response...

Look at Pass 350.8 or 250.8 very dynamic with  great  staging  and bass also a great co.

What is your budget ?and what kind of speakers ?

Cayin h80a amp. 80wpc of pure class a. 
Dual mono design. Can be had used for 1500 or so. Drives my mc-rl21s, which are 4 ohm, to sonic heaven.