Is it extra gain or two matched cables or both that improve the sound ?

I'm in the same situation.
I have an ability to switch my signal chain through Nakamichi RX202 tape deck. By engaging 'record' I can boost my preamp level by another 10dB.
I loose some transparency, but I gain weight. I mostly do that for parties.

I just switch tape/monitor depending on whether I am listening to a record or tape, of course. My Nak also has dual input control in addition to master control, so I can use it as a balance control, though the Redgum amp has dual volume control.
Thinking of possible upgrades, first I would probably replace the Colossus with the cable that has at least the same resolution and the weight of the Maximus. That would be older Purist Proteus or Dominus, or current Purist Poseidon, which is pure copper, or Aqueous, which is copper/silver/gold alloy. Next step would most likely be new integrated or possibly separates. But I am quite happy with how it is now. This Colossus ferox interconnect/Maximus ferox interconnect/Colossus fluid speaker cables chain is a great combination.

Replacing the phono with adjustable gain settings would make sense too, I guess. But this Acoustech is very good, I would have to go significantly higher to improve everything not just gain flexibility. I would think that the least expensive choice would be Sutherland 20/20 or whatever it is called. It's over $2k new, and try to find it used. Or tube phono, even higher cost.

Dear inna: """ So, overall the best sound is achieved when running phono/Colossus/deck/Maximus chain. """

NO, it’s not. It’s only what you think/like but not what it’s really happening.

The cartridge signal is really delicate and if we care about music our main target is to try that that audio signal stay at minimum contaminated by several distortions sources. We have to take care about mantain the integrity of what the cartridge pick up from the LP grooves.

That delicate and sensible cartridge signal is surrounded ( between other sources. ) by degradation signal at each single link all over the audio system chain.

You have to think that even the headshell/tonearm input connectors and the headshell/tonearm it self makes its works degrading the music/audio signal along the tonearm internal wires followed by IC cables between tonearm and phono stage. This IC cable makes a signal degradation at diferent levels/steps: its input connectors, the solder between those inputs connectors and the IC wires, the IC wires it self and again more connectors and solder at its output. Signal degradationj continues inside the phono stage at each single pasive and active parts where the signal must pass along all the switchs using there and through the volumen control too.

So at the ed of the audio chain we are listening a " fake/clown " of the original signal full of everykind of distortions you can or can’t imagine!!

So, what you like are those very high distortions signal degradations and what you don’t like is that more " direct " phono stage connection that comes with a lot of lower degradation/distortions. That " fullest sound " you mentioned are only higher distortions against lower distortions that is what you don't like it in the " direct " connection ".

At the end if that’s what you like stay there because the important subject is that you be happy.

Regards and enjoy the music,
R.