I think what you are experiencing with “At Fillmore East” is more complex than it appears. The easiest issue to address is the lack of impact or slam as you call it. Impact, is largely experienced in the 40 hz to 50 hz range, created mostly with the kick drum and to a somewhat lesser extent, the lowest three to four notes of the bass. A 5.5” woofer is just too small to deliver impact, They just cannot move enough air. To move air you need a large driver. I know you can achieve impact with a 12” driver, but I do not know if you can achieve it with a 10” or 8” driver.
As others have stated, adding a sub should add a lot to your system, though beware, adding a sub can bring great pleasure, but if not implemented correctly it can bring immense frustration. I would look into the Audiokinesis swarm. The swarm is designed to help eliminate a lot of the issues that powerful bass reproduction can cause. It probably is also, pound for the pound the best value. The one draw back might be that the swarm, I think, uses 10” drivers, but if Duke says that they will supply the impact you are looking for, I would trust him.
Now having said all of that, in my opinion there is not a lot of impact on “At Fillmore East”. The kick drums are pretty much berried in the mix, which was pretty typical for the era. The bass is more prominent, but the lowest notes, in my opinion are masked by a 3 to 6 dB bump between 125 and 160 hz and another bump at around 500 hz. If you have a way of removing those bumps, then the low bass becomes much more prominent and more of a dominate factor.
Other things that may help you achieve more impact would be to look into room treatments and if you decide to add a sub to your system, would be to eliminate the frequencies that sub is reproducing from your main speakers. This is something, I believe the swarm is designed to do but I’m not sure if it can do it with the Cronus.