How much do I need to spend to get a preamp that sounds better than no preamp?

Regarding Steve’s McCormack amplifiers, I believe all of them originally had a 100K ohm input impedance, up to the DNA-500, which was designed with a 10K ohm input impedance.

Yes these lower (10kohm) later amps will not be a good match for passives.
But! as the OP used it, direct from a 100ohm source to amps, no problems so long as the input sens is still 0.8v or even 2v for full output, it will to many still sound better than any active preamp, with or without gain.

Cheers George

I think it is worth mentioning that the OP's Audible Illusions L-1 preamp has a gain control and a master volume control. So one can set the perfect amount of gain for the amp and then use the master volume control there after.

Yes Audible Illusions are a very good active preamps, I owned an AI 3A and loved it.
They did run the tubes very hard in it though, you don’t get much life out of them, I’ve always believed that’s why it sounds as good as the best active preamps I’ve heard.

But the OP and me, going direct sounds the best for transparency and dynamics, and to me and many others a well implemented passive comes in a very close second.
To me any active stages add colouration's and cannot improve on the dynamics against a well implemented passive, they can only detract as they have no dynamic range expander circuit in them, and they wouldn't want to as they sound rubbish. 

Cheers George

I would like to know the proper way to use those gain controls. Audible Illusions is not much help.

honashagen OP
 I would like to know the proper way to use those gain controls. Audible Illusions is not much help.

I would do it so the "master volume control" is around 3 o’clock for normal listening level. That gives you leeway up or down for louder listen or quieter recorded music.

Cheers George