Increasing bass on floor standing speakers

Not sure about your loudspeakers but a 40WPC Rotel isn't going to have kick ass bass no mater what. 

You may find it useful to download a Real Time Analyzer (RTA) to your phone and check your room for bass modes and nulls--then you will see if your listening position is in a null.  I use AudioTools but there are many others.  You can also use a free room mode calculator called Amroc, but that's more helpful for modes rather than nulls.

Don't know if it's been mentioned, but you could try placing them on soft footers (I usually recommend first trying something from around the house like cutting up an old mouse pad/rubber mat, et cetera).

Does the Rotel's LF boost (6dB @ 100Hz) help?

As far as moving the speakers goes sometimes just a few inches will make a difference.

DeKay

These look the closest however there is a slit in the bottom at the front. https://impactaudio.co.uk/collections/visaton-loudspeaker-kits-accessories/products/visaton-vib-2000...
I have the ortofon 2m red cartridge (new). 
I also have cd player (nad) and a computer attached. 
I'll try moving the speakers but it's not a practical solution unfortunately and I'll do a test with tilting them. The bass is also light with the cd player. 

Do you have another music source to try, aside from the TT?

It could be the TT/cartridge setup.

Also, what cartridge?

I asume that you meant Visaton and if so see if anything looks similar here in order to let us know what general type of speaker you have.

https://impactaudio.co.uk/collections/visaton-loudspeaker-kits-accessories#.VdoIMZdU4Q0

DeKay

Does tilting the speakers help improves bass?

If the aforementioned suggestions do not yield the results needed, add a subwoofer. I recently ran into a similar issue. Found a little subwoofer at the thrift store and added it. Big improvement. Found a bigger better subwoofer on c.l. for a song. Daisy chained it together and it sounds much better. $60 and a couple sets of interconnects that were boxed up and problem resolved. Good luck!

If you can, put speakers on the short wall, not the long wall. Start there. Find listening position with the best overall response. Tweak from there.

I'm a novice although I purchased my first equipment in the 70s while overseas in the military. I now merely have a Yamaha N803 receiver and Vandersteen 1B speakers that are about 25 years old. I stream Spotify and have it set too the highest quality. I will probably upgrade but for now, I'm pleased that I kept the cost down, and most of my listening is background. I'm considering new speakers, probably from World Wide Stereo in Hatfield, Pa. near my home.  I heard and like their Focals.  Would I be wasting money with such a modest receiver or could I improve my sound considerably with new speakers.  Thanks for any advice.   Doug Hess, Newtown Square, Pa.

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 🇿🇦 

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

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. 

to cheer you up, from that album

Rocket Love, by Sophie Milman

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

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.

Upgrade your speaker cables could make a big difference on bass. They did on mine. 

Two suggestions:
1. Mount your floorstanders on isoacoustics Gaia feet. 2. Add (REL) subwoofers. 

Thanks everyone... I've got a few things to try. I noticed that the cable used to wire them was pretty thin so I'll fix this up when I check if they are in phase or not. 

I wrote a lot about getting more bass here:

https://speakermakersjourney.blogspot.com/2020/04/how-to-not-buy-subwoofer.html

Walls. Proximity to walls is the single biggest factor. Both for the speakers as well as for you. Speakers back against a wall and near the floor with you sitting near a wall, that's about as much bass as you're ever gonna get. Usually people prefer to move them around trying to get smooth and even, and for that it helps to bring things out into the room more.  

Another thing, what I described is very general. But bass isn't one thing its a range and a room isn't one thing either there's different shapes. It may be you are sitting in an area of suckout or cancellation. So try listening closer or further away if you can and see what that does.

Double check and make sure the speakers are wired in phase, using appropriate size cable. Move the speakers closer to the walls or corners.

Thanks for your reply. 
It's a kitchen and lounge long room with a hallway door at the other end from where I listen. The speakers are set up on the long wall 2.1metres apart starting 1.6m from the corner of the room. I sit opposite them 2.1metres away. 
Yes it's a relative thing... Too much mid and highs for the bass. Just using my ears to assess this and comparison to some tannoy m1 bookshelf speakers. 

How did you determine that the bass is a little weak? Is it the balance of the mids to highs too much relative to the bass? Are there four walls or is one end of the long room open to another room? You may need to describe the room in more detail and where the speakers are in the room, as well as where you sit to listen.

Let us know what you've already tried to determine your bass lack, and anything you think of trying to correct the situation.