Rega Planar 3 : Heavily upgraded yet produces a sound that I don't like


I have the Rega Planar 3 turntable for 30 years now. It had an original everything including a 100$ nottingham analogue cartridge/stylus.
Sound was not bad but pretty basic....
A few years ago I started upgrading it. I did all the upgrades together and the result is... not good. The sound is very accurate but it is too bright, there is almost no Bass and drums have no attack whatsoever.
I can't eliminate where the problem is because all upgrades are irreversible.
I know that the Ortofon Rondo Red has a very bright sound but still...

 

Here is a list of the upgrades :

1. ISOkinetik ISOvert Rega Tonearm VTA Adjustment Kit

https://www.analogueseduction.net/isokinetik-upgrades-and-parts/isokinetik-isovert-rega-tonearm-vta-adjustment-kit.html

 

2. Cardas Litz purple phono cable

 

3. ISOkinetik Tonearm Low CG weight

https://www.analogueseduction.net/isokinetik-upgrades-and-parts/isokinetik-isoweight.html

 

4. Ortofon Rondo Red Cartridge/stylus

 

5 Linn Linto phono pre amp (an old but outstanding piece which has a cult following).


6. I use Rega Elicit Apmplifer


triskadecaphobic
Do you have an external tracking force gauge?  Could be a cartridge / arm compatibility issue.  The Rondo Red is fairly low compliance and may not be well suited to your fairly low mass tone arm. This could be impacted bass response.
You changed the cables and might have a capacitance issue. MM cartridges are very sensitive to capacitance and the wrong capacitive loading will change the high frequency balance. You have three variables you can play around with, the cartridge, the resistance load and the capacitance load. You can "tune" your high frequency response this way.
I always shoot for the lowest capacitance in cables and usually play around with resistors first. Your load now is probably 47K ohms. Read this article https://www.tnt-audio.com/sorgenti/load_the_magnets_e.html
A word of caution. When people note a bright sound this usually means a peak between 6 kHz and 12 kHz. Most people will not hear a peak at 20 kHz. Even a peak at 16 kHz might not be perceived as "too bright" but rather as more detail which some people like. It gives you that surrealistic Hi Fi sound. 
I had a Rega Planar 3 back in the mid 90’s.  It had a Linn K9 cartridge.  Sound was very lightweight and listener fatigue stopped me from buying records.   I ended up keeping the Rega RB300 arm from the Planar3 and bought a VPI Hw19.  I mounted the RB300 and Linn cartridge and there was bass again!  Sold the Rega TT less arm and was happy for a time. 
I would unload the TT here and get something else with the money.  
I am happy with the Elys II cartridge on my Planar 3.
I realize at $295 it might not be what you want to spend.
However I do think the Rega cartridges setup easily and are meant specifically for their TT so for me they just work. You’ll get various opinions for sure.