Increasing bass on floor standing speakers

Did they ever sound strong? not any more?

Have you checked to see if the woofer’s cones and/or surrounds are in good shape? Look for pin holes, press in, check cone and surround stiffness: see if ’spring’ action feels good or kind of weak.

I have re-coned my 15" woofers (made in 1958) twice. They sound good, but I will be checking them soon, it’s been years.

I have to remove the back panel, then remove the driver from the front panel, check them out.

to cheer you up, from that album

Rocket Love, by Sophie Milman

https://www.youtube.com/watch?v=A2YO1DnRQBg

I used a polarity tester and the woofers are on the same phase. The tweeters were giving inconsistent results... One alternated with every pulse, the other was a little more random. I assume I wasn't using the app properly (I played through one phone connected to the amp and received with another. Used different frequencies and the L R fader.)
I'll rewire it with thicker internal wires when I know if I need to change any polarities. 

The speakers are new to me so I'm not sure if the sound has changed over time. 

Speakers on the long wall, I'd guess not much space behind the listening position then? 

Sounds like a perfect case for bass (wave) cancellation. 
If not quite that, figure out about any 'other' unintended bass traps in your listening area. 

I had this issue and only found out once I moved to another location. It can be subtle but annoyingly effective.
I know now. 
Bass problem, in the new location (speakers on short wall), gone like magic. 
Michélle 🇿🇦