Preamps waste of money?

Your case is one of a line of particular cases where you have sufficient input sensitivity for the rated output power and sufficient high input impedance.
In many other cases preamp might be a necessary component.
Try out simple active preamp with low gain instead of your resistor such as McCormack or Wyred4Sound and you may realize the improvement since you will regulate output voltage rather than output impedance of your passive component.

It`s rare when a resistor based VC is found to be less colored than LDR or transformer VC.Usually it`s the other way around.Resistors are said to have more of a signature according to many who`ve compared different types.There`ll always be exceptions to any generalization and each system is unique in terms of outcome.
Regards,

I am now using the Townshend Allagri transformer coupled passive and it is the most uncolored "preamp" I have had. But good transformer coupled ones are not cheap; it is $3000 and the top end over $12,000.

My experience is the polar opposite of yours. I find an active preamp to be the heart of a good system. Any and all attempts to use a passive or volume control on the amp or source have come up way short in my experience.

In my experience to not have a good active preamp is somewhat of a hindrance to great sound and music.

All of this is so subjective and it makes perfect sense you may think the polar opposite of myself.

Enjoy your music as you like!