cboshuizen
I've been enjoying SK10 lately. 
https://www.wavesfactory.com/product/sk10/ 
It's free. 

I'd never heard of SK10, I have done a bit of research and it looks good. I will download that later today. Many thanks.

Thank you, i've not tried the percussion strip on a bass track, i will give that a go..
 

I had a standalone copy of Maxxbass years ago, on a disc. Disc long gone! 

JonD
http://nickfever.com/music/blog/2015/bass-enhancer-vst-or-maxxbass-alternative
 
Also, there's the freebie Sanford Bass Tightener.
 
Or, you can get the original (or similar) from Waves.  Waves Maxx Bass, Renaissance Bass, and Lo Air are all on sale now for $29 each.
 
Some retailers discount further -- for example, everyplugin's price is $24.29.  (You'll need to register first).

Yeah, you're right, the current waves sale is making thinks pretty good value for money. I've just forked out on some new studio monitors so I may have to wait a while before i make some plugin purchases though!
 
Thanks for the tip off about the sanford bass plugin - i will give that a try.

Some cool freebies, thanks! I know I'm sounding like a broken record by now, but SK10 does the opposite of MaxxBass. It's probably the name of the plug that makes one assume it adds bass, when in effect it adds upper harmonics.

Sanderxpander
Some cool freebies, thanks! I know I'm sounding like a broken record by now, but SK10 does the opposite of MaxxBass. It's probably the name of the plug that makes one assume it adds bass, when in effect it adds upper harmonics.

Sorry, meant to say earlier in the day, youre explanation / difference of maxxbass and the harmonics was really useful. I understand that now, thank you.

What about Boz Bark of Dog? It's a freebie and should already be included with Prochannel.

Take a look at Meldas MBassador! It can recreate lost frequencies, and also manipulate different harmonics and octaves on the bass signal. You have to fix the "dynamics" with other plugins, but to restore/enhance the bottom end, I find it very useful!
https://www.meldaproduction.com/MBassador
 

Ha, I haven't tried MBassador but from the description at least one of its modules sounds closer to MaxxBass than all the others that have been mentioned.
It's much pricier than the Waves ones (on sale) though.

Sanderxpander
It's much pricier than the Waves ones (on sale) though.

Melda plug-ins go on sale for 50% off regularly, are true VST2 and VST3 plug-ins, have free updates for life, no WUP and simple serial number protection.